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This CL moves most of the actual work of initializing compositor thread
from RenderThreadImpl to Platform. This further reduces //content's
dependency to WebThread.

A new virtual function is introduced to Platform so we can change the
thread priority of the created compositor thread on Linux, where we need
to talk to the browser process to change the thread priority.

Additionally, a new function Platform::CompositorThreadTaskRunner() is
introduced, because RenderThreadImpl needs to post tasks to the
compositor thread after the thread is initialized.

Bug: 826203
Change-Id: Ic80e6452cdfe8af25ae2b277125687854859d106
Reviewed-on: https://chromium-review.googlesource.com/1249424
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@{#595063}