[Merge to M58] CrOS: Do not allow notifications to be added during shutdown.

chrome/browser/notifications/notification_ui_manager.h

Index: chrome/browser/notifications/notification_ui_manager.h
diff --git a/chrome/browser/notifications/notification_ui_manager.h b/chrome/browser/notifications/notification_ui_manager.h
index 7bee24c621db0a4319211d28d52920be4c2269c5..9fdb05ab85e90b57c861e90d465decb5a418703f 100644
--- a/chrome/browser/notifications/notification_ui_manager.h
+++ b/chrome/browser/notifications/notification_ui_manager.h
@@ -86,6 +86,11 @@ class NotificationUIManager {
   // Used when the app is terminating.
   virtual void CancelAll() = 0;
 
+  // Cancels all pending notifications and closes anything currently showing.
+  // After this is called, no new notifications can be added. Used when the app
+  // is terminating.
+  virtual void StartShutdown() = 0;
+
  protected:
   NotificationUIManager() {}