New SequenceManagerImpl methods to support MLC and STE

The new methods will let us support MessageLoopCurrent and
ScopedTaskEnvironment.

Bug: 863341, 891670
Change-Id: I1358330e890e0f669de2869c3d9ee2255318e422
Reviewed-on: https://chromium-review.googlesource.com/c/1326012
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606542}
12 files changed