Fix message_center::MessageView::OnSlideOut UAF

Bug: 983009
Change-Id: Ie285c4035bfc4c45e27e5f3fd6c34280d9f4c2d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2001943Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Matthew Denton <mpdenton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732131}

ui/message_center/views/message_view.cc

@@ -348,11 +348,12 @@ void MessageView::RemoveSlideObserver(MessageView::SlideObserver* observer) {
 }
 
 void MessageView::OnSlideOut() {
-  MessageCenter::Get()->RemoveNotification(notification_id_,
-                                           true /* by_user */);
-
   for (auto& observer : slide_observers_)
     observer.OnSlideOut(notification_id_);
+
+  // RemoveNotification() may delete |this|.
+  MessageCenter::Get()->RemoveNotification(notification_id_,
+                                           true /* by_user */);
 }
 
 void MessageView::OnWillChangeFocus(views::View* before, views::View* now) {}