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This change removes a double slide animation when the user flings to
dismiss a notification.

TEST=manually verified, new unit test
BUG=933640

Change-Id: I7f9f4e558dfa53e8acf0abdbd86cf4449c1ea891
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1583344
Commit-Queue: Tim Song <tengs@chromium.org>
Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654564}