Simplify SequenceManager construction

We move the iOS / Android AttachToMessagePump method internally and
change sequence_manager::CreateUnboundSequenceManager to create a
ThreadControllerWithMessagePumpImpl since the message loop variant is
unused.  It's still possible to create a SequenceManager on top of
the current MessageLoop via CreateSequenceManagerOnCurrentThread.

Bug: 872372, 863341
Change-Id: I7459c29b5cf80a47720ae9781347bf55c7c75ac0
Reviewed-on: https://chromium-review.googlesource.com/c/1458197
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Auto-Submit: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630403}
11 files changed