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This will used for a perf test. Adds an api and a supporting animation
observing class which waits until the animation has been scheduled,
which means the desk ending screenshot has been taken. The observing
class then activates the adjacent desk. This repeats until we have
reached the target desk, then we wait for the whole animation to finish
and run the given callback.

Test: added test
Bug: 1111445
Change-Id: I9f859a2ae31da958c0d539f7e00a1a9944c3bb3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437151
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815409}