CreateUnboundSequenceManager to not require BindToMessageLoop

By mistake SequenceManagerImpl is crashing on exit if
CreateUnboundSequenceManager(null) is called but BindToMessageLoop
is not.  This patch fixes that.

Bug: 863341, 872372
Change-Id: I2827ab40c1e32553f1c42229762732b5cd42801d
Reviewed-on: https://chromium-review.googlesource.com/1251448
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595093}
3 files changed