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This animation was removed unintentionally, this CL brings it back.
Also added better comments so the animation's importance is more
obvious.

Bug: 910496
Change-Id: If505bea1683d7ae456dcedbdb0a9e97ea58e02de
Reviewed-on: https://chromium-review.googlesource.com/c/1368604Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Reviewed-by: Mitsuru Oshima (gardener - slow) <oshima@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616080}