Add message to Ash message center for no notifications

ash/system/web_notification/web_notification_tray.h

Index: ash/system/web_notification/web_notification_tray.h
diff --git a/ash/system/web_notification/web_notification_tray.h b/ash/system/web_notification/web_notification_tray.h
index 73f39c7f57390dbb457722e15e99e7c6494de748..880123c5f3639f5bcb629eca4de139f2f0f65ad5 100644
--- a/ash/system/web_notification/web_notification_tray.h
+++ b/ash/system/web_notification/web_notification_tray.h
@@ -110,6 +110,9 @@ class ASH_EXPORT WebNotificationTray : public internal::TrayBackgroundView {
   // Hide the message center bubble. Should only be called by StatusAreaWidget.
   void HideMessageCenterBubble();
 
+  // Hide the message center bubble if there are no notifications.
+  void HideMessageCenterBubbleIfEmpty();
+
   // Show a single notification bubble for the most recent notification.
   void ShowNotificationBubble();