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This CL moves the ownership of the compositor thread from RenderThread-
Impl to Platform in order to reduce //content's dependency to
WebThread.

Now Platform::CompositorThread() is a concrete member function, and
the compositor thread is initialized by Platform::Initialize-
CompositorThread(). Some components of //content require the task
runner of the compositor thread, so RenderThreadImpl::
compositor_task_runner() is still available.

Bug: 826203
Change-Id: I6fe71835d7b7671d8781744ec62561242c6be29f
Reviewed-on: https://chromium-review.googlesource.com/1245039
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594611}