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This CL implements animation for clearing all Notifications.

This CL does not accurately implements the animation as the spec. For
the UX spec, see the bug.

TEST=UnifiedMessageListViewTest
BUG=897915

Change-Id: I750b3a3e9d86706bef7c9cd13474dbff986486e9
Reviewed-on: https://chromium-review.googlesource.com/c/1312176Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609203}