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This CL adds workaround to MessageView::SlideOut() for a bug that it did
not work with MessagePopupCollection.

If |by_user| of RemoveNotification is true in SlideOut(),
IncrementDeferCounter() in MessagePopupCollection::OnNotificationRemoved
is called but corresponding DecrementDeferCounter() won't be called.

MessagePopupCollection will be completely rewritten.

TEST=message_center_unittests
BUG=805208

Change-Id: I1a637cef8bdcb25be45647f487f1d29ab3952583
Reviewed-on: https://chromium-review.googlesource.com/952822Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542686}