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This is a step in the direction of better compartmentalizing each
shelf component and better coordinate animations. It also aims at making
the shelf layout manager not so huge.

Change-Id: I7369e00f87bda1fac0956e119d60f34d9716a651
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033567
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737572}