This patch reworks image decompression and collection in the following ways
because of misbehavior in the described edge cases.

The current flow for realizing a texture on the GPU from a blob of compressed
bytes is to first pass it to the IO thread for image decompression and then
upload to the GPU. The handle to the texture on the GPU is then passed back to
the UI thread so that it can be included in subsequent layer trees for
rendering. The GPU contexts on the Render & IO threads are in the same
sharegroup so the texture ends up being visible to the Render Thread context
during rendering. This works fine and does not block the UI thread. All
references to the image are owned on UI thread by Dart objects. When the final
reference to the image is dropped, the texture cannot be collected on the UI
thread (because it has not GPU context). Instead, it must be passed to either
the GPU or IO threads. The GPU thread is usually in the middle of a frame
workload so we redirect the same to the IO thread for eventual collection. While
texture collections are usually (comparatively) fast, texture decompression and
upload are slow (order of magnitude of frame intervals).

For application that end up creating (by not necessarily using) numerous large
textures in straight-line execution, it could be the case that texture
collection tasks are pending on the IO task runner after all the image
decompressions (and upload) are done. Put simply, the collection of the first
image could be waiting for the decompression and upload of the last image in the
queue.

This is exacerbated by two other hacks added to workaround unrelated issues.
* First, creating a codec with a single image frame immediately kicks of
  decompression and upload of that frame image (even if the frame was never
  request from the codec). This hack was added because we wanted to get rid of
  the compressed image allocation ASAP. The expectation was codecs would only be
  created with the sole purpose of getting the decompressed image bytes.
  However, for applications that only create codecs to get image sizes (but
  never actually decompress the same), we would end up replacing the compressed
  image allocation with a larger allocation (device resident no less) for no
  obvious use. This issue is particularly insidious when you consider that the
  codec is usually asked for the native image size first before the frame is
  requested at a smaller size (usually using a new codec with same data but new
  targetsize). This would cause the creation of a whole extra texture (at 1:1)
  when the caller was trying to “optimize” for memory use by requesting a
  texture of a smaller size.
* Second, all image collections we delayed in by the unref queue by 250ms
  because of observations that the calling thread (the UI thread) was being
  descheduled unnecessarily when a task with a timeout of zero was posted from
  the same (recall that a task has to be posted to the IO thread for the
  collection of that texture). 250ms is multiple frame intervals worth of
  potentially unnecessary textures.

The net result of these issues is that we may end up creating textures when all
that the application needs is to ask it’s codec for details about the same (but
not necessarily access its bytes). Texture collection could also be delayed
behind other jobs to decompress the textures on the IO thread. Also, all texture
collections are delayed for an arbitrary amount of time.

These issues cause applications to be susceptible to OOM situations. These
situations manifest in various ways. Host memory exhaustion causes the usual OOM
issues. Device memory exhaustion seems to manifest in different ways on iOS and
Android. On Android, allocation of a new texture seems to be causing an
assertion (in the driver). On iOS, the call hangs (presumably waiting for
another thread to release textures which we won’t do because those tasks are
blocked behind the current task completing).

To address peak memory usage, the following changes have been made:
* Image decompression and upload/collection no longer happen on the same thread.
  All image decompression will now be handled on a workqueue. The number of
  worker threads in this workqueue is equal to the number of processors on the
  device. These threads have a lower priority that either the UI or Render
  threads. These workers are shared between all Flutter applications in the
  process.
* Both the images and their codec now report the correct allocation size to Dart
  for GC purposes. The Dart VM uses this to pick objects for collection. Earlier
  the image allocation was assumed to 32bpp with no mipmapping overhead
  reported. Now, the correct image size is reported and the mipmapping overhead
  is accounted for. Image codec sizes were not reported to the VM earlier and
  now are. Expect “External” VM allocations to be higher than previously
  reported and the numbers in Observatory to line up more closely with actual
  memory usage (device and host).
* Decoding images to a specific size used to decode to 1:1 before performing a
  resize to the correct dimensions before texture upload. This has now been
  reworked so that images are first decompressed to a smaller size supported
  natively by the codec before final resizing to the requested target size. The
  intermediate copy is now smaller and more promptly collected. Resizing also
  happens on the workqueue worker.
* The drain interval of the unref queue is now sub-frame-interval. I am hesitant
  to remove the delay entirely because I have not been able to instrument the
  performance overhead of the same. That is next on my list. But now, multiple
  frame intervals worth of textures no longer stick around.

The following issues have been addressed:
* https://github.com/flutter/flutter/issues/34070 Since this was the first usage
  of the concurrent message loops, the number of idle wakes were determined to
  be too high and this component has been rewritten to be simpler and not use
  the existing task runner and MessageLoopImpl interface.
* Image decoding had no tests. The new `ui_unittests` harness has been added
  that sets up a GPU test harness on the host using SwiftShader. Tests have been
  added for image decompression, upload and resizing.
* The device memory exhaustion in this benchmark has been addressed. That
  benchmark is still not viable for inclusion in any harness however because it
  creates 9 million codecs in straight-line execution. Because these codecs are
  destroyed in the microtask callbacks, these are referenced till those
  callbacks are executed. So now, instead of device memory exhaustion, this will
  lead to (slower) exhaustion of host memory. This is expected and working as
  intended.

This patch only addresses peak memory use and makes collection of unused images
and textures more prompt. It does NOT address memory use by images referenced
strongly by the application or framework.

Added class docstrings for classes inside of shell/common.

Using it, a Flutter app can monitor missing frames in the release mode, and a custom Flutter runner (e.g., Fuchsia) can add a custom FrameRasterizedCallback.

Related issues:
https://github.com/flutter/flutter/issues/26154
https://github.com/flutter/flutter/issues/31444
https://github.com/flutter/flutter/issues/32447

Need review as soon as possible so we can merge this before the end of May to catch the milestone.

Tests added:
* NoNeedToReportTimingsByDefault
* NeedsReportTimingsIsSetWithCallback
* ReportTimingsIsCalled
* FrameRasterizedCallbackIsCalled
* FrameTimingSetsAndGetsProperly
* onReportTimings preserves callback zone
* FrameTiming.toString has the correct format

This will need a manual engine roll as the TestWindow defined in the framework needs to implement onReportTimings.

This follows our namespace change from shell to flutter: https://github.com/flutter/engine/pull/8520.

Some components in the Flutter engine were derived from the forked blink codebase. While the forked components have either been removed or rewritten, the use of the blink namespace has mostly (and inconsistently) remained. This renames the blink namesapce to flutter for consistency. There are no functional changes in this patch.

Revert "Revert "Separate the data required to bootstrap the VM into its own class. (#8397)" (#8406)" (#8414)

This reverts commit f7b4903d.

This reverts commit c9916474.

When attempting to shutdown and subsequently restart the VM, having the
VM own this data introduces lifecycle issues due to circular references.

This reverts commit 0d6ff166.

The shell was already designed to cleanly shut down the VM but it couldnt
earlier as |Dart_Initialize| could never be called after a |Dart_Cleanup|. This
meant that shutting down an engine instance could not shut down the VM to save
memory because newly created engines in the process after that point couldn't
restart the VM. There can only be one VM running in a process at a time.

This patch separate the previous DartVM object into one that references a
running instance of the DartVM and a set of immutable dependencies that
components can reference even as the VM is shutting down.

Unit tests have been added to assert that non-overlapping engine launches use
difference VM instances.

* Make IOManager own resource context

The current implementation only deals with Android devices and we'll add iOS devices support soon.

Keep a copy of each engine's description that can be accessed outside the engine's UI thread (#6885)

The service protocol's ListViews method needs to return description data for
each engine in the process.  Previously ListViews would queue a task to each
UI thread to gather this data.  However, the UI thread might be blocked from
executing tasks (e.g. if the Dart isolate is paused), resulting in a deadlock.

This change provides a copy of the engine's description data to the
ServiceProtocol's global list of engines, allowing ListViews to run without
accessing any UI threads.

Fixes https://github.com/flutter/flutter/issues/24400

TL;DR: Offscreen surface is created on the render thread and device to host
transfer performed there before task completion on the UI thread.

While attempting to snapshot layer trees, the engine was attempting to use the
IO thread context. The reasoning was that this would be safe to do because any
textures uploaded to the GPU as a result of async texture upload would have
originated from this context and hence the handles would be valid in either
context. As it turns out, while the handles are valid, Skia does not support
this use-case because cross-context images transfer ownership of the image from
one context to another. So, when we made the hop from the UI thread to the IO
thread (for snapshotting), if either the UI or GPU threads released the last
reference to the texture backed image, the image would be invalid. This led to
such images being absent from the layer tree snapshot.

Simply referencing the images as they are being used on the IO thread is not
sufficient because accessing images on one context after their ownership has
already been transferred to another is not safe behavior (from Skia's
perspective, the handles are still valid in the sharegroup).

To work around these issues, it was decided that an offscreen render target
would be created on the render thread. The color attachment of this render
target could then be transferred as a cross context image to the IO thread for
the device to host tranfer.

Again, this is currently not quite possible because the only way to create
cross context images is from encoded data. Till Skia exposes the functionality
to create cross-context images from textures in one context, we do a device to
host transfer on the GPU thread. The side effect of this is that this is now
part of the frame workload (image compression, which dominate the wall time,
is still done of the IO thread).

A minor side effect of this patch is that the GPU latch needs to be waited on
before the UI thread tasks can be completed before shell initialization.

Format code from previous patch

* If isolate is already running, return true

* Use shell::Engine::RunStatus as result of Engine::Run

Following fixes in dart-lang/sdk@91cbb57cd5cd012d5aa66bfb6eb3a1306271f431

This reverts commit e3133e0e.

 - Add missing Dart 2 flags to gen_snapshot invocation.
 - Disable brittle service test.

This reverts commit 4c2448d1.

 - Switch core snapshot to Dart 2 and remove support for loading platform.dill.
 - Remove support for loading script snapshots.
 - Remove support for loading source.
 - Remove settings and fix names to reflect the above.
 - Remove support for loading the service isolate from source.

When these delegate methods were initially added, it was expected that a single
shell would be able to own mutliple platform views, engines and animators. This
plan was abandoned in favor of creating multiple shells with their own platform
views, engines, etc.. The arguments were meant to ease the disambiguate the
instances of the variaous objects managed by the shell. This is no longer
necessary.

* Flush all embedded Android view on hot restart.

Adds an OnEngineRestarted method to PlatformView, this is currently only
implemented for Android where we need to use it for embedded views.

* review comments followup

* rename to OnPreEngineRestart, call before Clone

Reverts flutter/engine#5740

Required by the Fuchsia embedder

* Reland "Added IsolateNameServer functionality (#5410)"

This reverts commit c3976b3c.

* Fixed issue with isolate_name_server_test which caused test to timeout

* Disabled thread_annotations on Android as they aren't supported in the
NDK headers for std::mutex. Readded thread annotations to
IsolateNameServer.

This reverts commit 61a2d129.

* Added IsolateNameServer functionality, which allows for the association
of string names with isolate SendPort ids that can be used to establish
inter-isolate communications.