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We're moving away from using MessageLoop in Blink, hence,
we want to make SequenceManager support any missing pieces
and provide a good level of abstraction on top of a thread.

Specifically, this CL will unblock removing MessageLoop
references from content::RenderThreadImpl.

Bug: 828835
Change-Id: I5a785a26f35839eea3705ecb5abc61672c497457
Reviewed-on: https://chromium-review.googlesource.com/1167516Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Greg Kraynov <kraynov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581876}