Ash Tray: Close quick settings bubble after pressing ESC in message center.

BUG=1027262

Change-Id: Id6745691e05f337a41de1aff2c5cbda558f85c14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1929617Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718777}

ash/system/message_center/unified_message_center_bubble.cc

@@ -200,8 +200,13 @@ void UnifiedMessageCenterBubble::OnWidgetDestroying(views::Widget* widget) {
   CHECK_EQ(bubble_widget_, widget);
   tray_->tray_event_filter()->RemoveBubble(this);
   tray_->bubble()->unified_view()->RemoveObserver(this);
+  message_center_view_->RemoveObserver(this);
   bubble_widget_->RemoveObserver(this);
   bubble_widget_ = nullptr;
+
+  // Close the quick settings bubble as well, which may not automatically happen
+  // when dismissing the message center bubble by pressing ESC.
+  tray_->CloseBubble();
 }
 
 }  // namespace ash