Add CHECK to make sure that code is only called from UI thread.

We are currently observing a crash in LoginHandler. See associated crbug. Our
theory is that this can only happen if a race conditions occurs due to code
running on a non-UI thread. Since the code in question should only ever run on
the UI thread, this CL adds a CHECK to confirm this theory.

Bug: 827169
Change-Id: I340d5ed7cc92be84305451a588d11e72fbac2990
Reviewed-on: https://chromium-review.googlesource.com/986478


Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546883}