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Fixes required for SequenceManager/ThreadControllerWithMessagePump to
work properly on all platforms:

Always call MessagePump::ScheduleWork from EnsureWorkScheduled to avoid
deadlocks on some platforms.

Switch from implicit quit_do_work to quit_pending, which is set to false
when the current run loop (and ThreadController::Run) call ends.

Improve test coverage.

R=alexclarke@chromium.org
СС=gab@chromium.org
BUG=891670

Change-Id: I9fb554ed91cdca6982e773f4b29055b4ae99dc95
Reviewed-on: https://chromium-review.googlesource.com/c/1337501
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608521}