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OnMessageCenterScrolled should be fired when it's scrolled by fling.
Otherwise, stacking notification counter https://crrev.com/c/1168945 and
notification corners https://crrev.com/c/1161710 might show invalid
state with touch screen.

TEST=manual
BUG=none

Change-Id: I0a647bdd5af979775ef4ef4e8c470a96cbfd299a
Reviewed-on: https://chromium-review.googlesource.com/1172233Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582863}