Move CompositorThread to a dedicated header.

This CL renames WebThreadForCompositor to CompositorThread, and moves
its definition to dedicated header and implementation files. This makes
it easier to locate its declaration.

Bug: 826203
Change-Id: I9ac8a48b22849adc1ae938f52827e665a96712ba
Reviewed-on: https://chromium-review.googlesource.com/c/1298825
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602665}
4 files changed