Per recommendation from the Skia team. Benefits from optimizations applied in Skia for Chrome.

We're now producing a flutter_content_handler binary for Fuchsia that
builds and links.

This required us to fix a backwards dependency from flutter/lib/ui to
sky/shell.

Also, remove the hacks that integrate Skia ref counting with WTF. Now we
just use sk_sp everywhere for managing Skia ref counts.

Fallback to creating Skia managed images on platforms that dont provide a GL context on the resource loader. (#2849)

This patch introduces a //glue library that isolates our dependency on //base.
This will let us clean up our //base dependencies ahead of actually being able
to fully remove them.

This patch removes almost all //base dependency of //flow. The only dependency
left is on tracing.

The code that draws the picture knows more about the picture (and the future)
than we do. This patch gives them a way to hint us about what's going to
happen with the picture in the future.

I noticed this while doing the libjpeg_turbo roll.

iOS: Discard the depth and stencil attachments of the framebuffer before presenting the color renderbuffer. (#2737)

* Added instrumentation for timings.

* A few more traces for better granularity.

This reverts commit 5bc5c7b5.

This commit appears to have caused
https://github.com/flutter/flutter/issues/3658

This lets Skia run an optimization pass over the SkPicture before actually
issuing GL commands.

Instead, make Layer::Paint take a new PaintContext that has just exactly
the state that it needs, mirroring PrerollContext. Also, rename
PaintContext to CompositorContext because it holds the context for the
whole compositor.

This patch makes these files better match Google C++ style.

Also, add the engine lap time when running on Mojo.

We now compute the bounds for all container layers, including clips, and
use those bounds to tighten down our clips and save layers.

Previously we were applying opacity in a different way than Skia
expects. Now we use the exact pattern that Skia expects so that we hit
more optimizations inside SkRecordOptimize.

After this patch, we don't actually get the optimization because we
don't yet run SkRecordOptimize over the composited tree. A later patch
will actually cause us to run SkRecordOptimize.

Particularly, add support for blur filters to implement backdrop blur effects.

The layout protocol in Mozart changed. Now we need to pipe a scene version
through the system to the compositor.

This required a bunch of changes because of the new sk_sp type Skia has
introduced into their API.

- fix dartdocs for addPerformanceOverlay
- make PerformanceOverlayLayer honour its x, y, and height.
- fix the y axis of PerformanceOverlayLayer to only show 3x16ms, since
  if it's more than 16ms it really doesn't matter what it is.
- make the label on the PerformanceOverlayLayer show the worst time on
  record not the instantaneous time.
- pin the fps to a maximum of 60Hz