Fix MessageListView crash on update during close.

MessageListView crashes when a notification is updated during the
UnifiedSystemTray is closing. Like other views::Widget methods,
SynthesizeMouseMoveEvent() should not be called during the widget is
closing.

See the bug for the asan trace.

TEST=manual
BUG=894070

Change-Id: I9ffba0f73260542ebf21b2aef572af1f68e5c40b
Reviewed-on: https://chromium-review.googlesource.com/c/1276225
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598768}

ash/system/message_center/message_list_view.cc

@@ -433,7 +433,7 @@ void MessageListView::OnBoundsAnimatorDone(views::BoundsAnimator* animator) {
   if (need_update)
     DoUpdateIfPossible();
 
-  if (GetWidget())
+  if (GetWidget() && !GetWidget()->IsClosed())
     GetWidget()->SynthesizeMouseMoveEvent();
 }
 

ash/system/message_center/new_unified_message_center_view.cc

@@ -96,7 +96,7 @@ void NewUnifiedMessageCenterView::ListPreferredSizeChanged() {
   PreferredSizeChanged();
   ScrollToPositionFromBottom();
 
-  if (GetWidget())
+  if (GetWidget() && !GetWidget()->IsClosed())
     GetWidget()->SynthesizeMouseMoveEvent();
 }