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Introdue a new overview observer which notifies observers when the start
animation for overview is different.

Uses will be both in and out of overview - for example adding mask, shadow
and animating tray, shelf.

Also modify CleanupObserverTest to use some new stuff.

Test: ash_unittests StartAnimationObserverTest.Basic
Bug: 898222
Change-Id: I86bdd253f0a4a4dd81ad6ba03780baf07923d256
Reviewed-on: https://chromium-review.googlesource.com/c/1297405
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602941}