Let SequenceManager bind to MessageLoopCurrent

This is a step towards introducing a UIThreadScheduler which will
have a SequenceManager rather than a MessageLoop on the UI thread.

We also remove the SequenceManager from base::Thread since we no longer
considering having anything other than a FIFO scheduler there.

TBR=marq@chromium.org

Bug: 863341, 891670
Change-Id: If1603e623688a5bb4d0ca71b2b55bc50ffb9c506
Reviewed-on: https://chromium-review.googlesource.com/c/1320329
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608060}
26 files changed