This patch moves the rest of the clients to use the new PointerData
transport.

* Namespaces have been updated to reflect the move from //flutter/sky/shell to //flutter/shell.
* shell/BUILD.gn file has been split into smaller GN files for each subcomponent of the shell (common, GPU, diagnostic, testing).
* GN dependencies have been rewritten to stop exposing common shell dependencies as public. Duplicates have also been removed.
* GPU subcomponent has been updated make it more suitable for Vulkan integration.
* The GLFW backend has been resurrected.

This patch hides the base::SingleThreadedTaskRunner, which pulls a big
switchover to ftl::WeakPtr and std::function.

Remove dependencies on ui, third_party/mesa and mojo/convertors. Provide simple alternatives. (#2783)

This patch switches us to building Runner.app on the end-developer's machine,
which will eventually let them customize their app.

Fixes https://github.com/flutter/flutter/issues/3256

SkyShell now supports --packages instead of --package-root.

These services are relative to the current view, which means they should come
in through the view services.

This patch cleans up the way we expose Mojo services into Dart. It also adds a
service provider for "view services," which will evetually contain the raw
keyboard service.

Previously this value was the temp directory that we used to store the FLX
bundle. Now it's the URL given to us by MojoShell.

Allow specification of the FLX bundle (containing no Dart snapshot) outside the iOS/Mac application bundle.

Allows launching of apps without having any Xcodebuild step in the simulator.

We now use std::unique_ptr more consistently.

Also, remove the --snapshot flag because using FLX files is the path we're
paving.

The time_stamp is supposed to be in microseconds. Previously we were using
milliseconds.

Now the control flow for wiring up platform services starts in the
platform-specific code. Previously we started in shared code, which was
imposing constraints on the startup sequence for all platforms. Now that we
start in platform-specific code, we'll be able to add better support for Mojo
in a future patch.

We weren't using the network service for anything.

This patch prepares us to add support for Mozart.

This is needed for coordination with sky_tools development.

Previously we defaulted to loading from URLs, but its
more common to want to load files off the local machine
on the command line.

Now you can use --snapshot to load snapshots from the command line and have
them appear visually on Mac. Also, remove code duplication with Linux by
factoring common testing code into //sky/shell/testing.

@chinmaygarde