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Currently, the notification bar abruptly disappears after the second to last
notification is dismissed, which is quite jarring.

This same animation will also be used when the clear all animation is updated to
spec.

Video:
https://screencast.googleplex.com/cast/NDg3MjczMzQ3MzExMjA2NHw3ZmNmNGY5OC01Yw

TEST=manually verified, updated unit test
BUG=958276

Change-Id: I55bfeb4b23c1c2bda47ccd21dbef5a5955a62675
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1592701
Commit-Queue: Tim Song <tengs@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658683}