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Reuse the existing ShelfAnimationWaiter and generalize it a little bit.

TBR: mmourgos
Bug: 1050788
Change-Id: Ica852940cb765143c242c2fa3f101c28cca6922a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2051025Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740458}