Null check control buttons view in OnSlideChanged.

In ARC++ notifications, GetControlButtonsView() could return null
during animation i.e. copied surface is shown.
This crash happens when an already swiped notification is swiped again.

TEST=manual
BUG=918936

Change-Id: I706765fb15ecf1fdabfc00692eb9f63405011dc0
Reviewed-on: https://chromium-review.googlesource.com/c/1404895
Reviewed-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621546}