overview: Add a test to verify that OverviewObserver calls match.

So we can safely use this assumption that start/end calls are 1:1. See
crrev.com/c/2092691 for motivation.

Test: ash_unittests OverviewControllerTest.ObserverCallsMatch
Bug: 1059454
Change-Id: I6e99c1775cdb1c2f4c1f1bc8d11c9b138858f623
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097160
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#749413}