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The old animation has a lot going on, it is hard for it to do well on
slower devices. In order to make these transitions look good on all
devices, change them a bit.

The new animation:
 - Shelf background snaps away for the duration of the animation.
 - Animating background animates up/down to be in its final position.

Bug: 1027751
Change-Id: If02f7a3f9d5f9ef9af59cf73747a9b7fab582042
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1930693
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719349}