Add TaskType::kWorkerThreadTaskQueueCompositor and use it for compositors

Before this CL, null was passed to Mojo binding as a compositor task
runner in offscreen canvas on worker thread as NonMainThreadTaskRunner::
CompositorTaskRunner() returns null. In this case,
SingleThreadTaskRunner::Get() was used internally Mojo.

This CL implements WorkerThreadScheduler::CompositorTaskRunner() with
a new task type kWorkerThreadTaskQueueCompositor so that we can know the
task runner's usage on UMA for task duration per task type.

Bug: 841172
Change-Id: I8caf3244169300aed45eeab4fcd01999bc823976
Reviewed-by: Kentaro Hara <>
Reviewed-by: Alexander Timin <>
Commit-Queue: Hajime Hoshi <>
Cr-Commit-Position: refs/heads/master@{#561847}
10 files changed