Reland: Implement and test task runners in RasterWorkerPool.
This CL moves RasterWorkerPool outside render_thread_impl.h and
in its own file, implements TaskRunner and SequencedTaskRunner
on top of it and properly tests the two implementations.
BUG=
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
Committed: https://crrev.com/
6526c209494a5176f82b5f400fd1e66a426c840f
Cr-Commit-Position: refs/heads/master@{#341827}
Review URL: https://codereview.chromium.org/
1254003003
Cr-Commit-Position: refs/heads/master@{#341946}