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

chrome/browser/notifications/message_center_notification_manager.h

Index: chrome/browser/notifications/message_center_notification_manager.h
diff --git a/chrome/browser/notifications/message_center_notification_manager.h b/chrome/browser/notifications/message_center_notification_manager.h
index 7e7f24dde421ca1143866eb24264635518228488..bb423f55277de2b1561d80a90964be79650fc8cb 100644
--- a/chrome/browser/notifications/message_center_notification_manager.h
+++ b/chrome/browser/notifications/message_center_notification_manager.h
@@ -60,6 +60,7 @@ class MessageCenterNotificationManager
   bool CancelAllBySourceOrigin(const GURL& source_origin) override;
   bool CancelAllByProfile(ProfileID profile_id) override;
   void CancelAll() override;
+  void StartShutdown() override;
 
   // MessageCenterObserver
   void OnNotificationRemoved(const std::string& notification_id,
@@ -112,6 +113,9 @@ class MessageCenterNotificationManager
   // Keeps track of notifications specific to Google Now for UMA purposes.
   GoogleNowNotificationStatsCollector google_now_stats_collector_;
 
+  // Tracks if shutdown has started.
+  bool is_shutdown_started_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(MessageCenterNotificationManager);
 };