Store the current SequenceManagerImpl instance in TLS

Now that the backend for MessageLoop is always a SequenceManager it no
longer makes sense to have the TLS logic in MessageLoopCurrent.

Eventually MessageLoopBase will disappear completely and
MessageLoopCurrent will use the API exposed by SequenceManager. This
patch is a step in that direction.

Bug: 891670
Change-Id: I24a76ded54fcf4b95bc8192ce0fe413aaeee162d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497074
Commit-Queue: Carlos Caballero <carlscab@google.com>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641834}
4 files changed