* This adds rpc call to simply set asset path.

This is needed when doing hot reload to pick up updated assets.

* Move asset_directory fetch for after view_id. Clean up return. Fix formatting.

* Add SetAssetBundlePath methods implementations for mac and ios

* Fix mac mm

* Fix formatting

* Merge and use nullptr

This will replace the one in the diagnostic server

In https://github.com/flutter/engine/pull/3833 the `_flutter.listViews` RPC moved from thread based to lock based synchronization.
The thread based synchronization side effect was used by flutter benchmarks in https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/vmservice.dart#L1223 and
https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/run_hot.dart#L156 to ensure the completeness of the restart/reload and so correct timing.

A new RPC `_flutter.flushUIThreadTasks` is introduced to allow the flutter benchmarks to reintroduce thread based synchronization.

Related https://github.com/flutter/flutter/issues/11241

This is intended for use in flutter_driver tests

* 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.

Add collection of PlatformViews to the Shell and allow them to be queried over the service protocol (#2861)