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Adds a drop-in motion to the current fade-in effect. This (fade in and
motion) is currently linear but expected to be updated in a later change
after consulting UX.

Bug: chromium:903806
Change-Id: Iebc44c281ad18be2c02a1e84903cc6d0ff64b84f
Reviewed-on: https://chromium-review.googlesource.com/c/1352689Reviewed-by: Sidney San Martín <sdy@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611475}