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 589633e44c7c4911dbd6589a88fc562d7c6a2a32..67063b1f314018999ac8e841260e00d9c98deae8 100644
--- a/chrome/browser/notifications/message_center_notification_manager.h
+++ b/chrome/browser/notifications/message_center_notification_manager.h
@@ -59,6 +59,7 @@ 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,
@@ -108,6 +109,9 @@ class MessageCenterNotificationManager
   // Keeps track of all notification statistics for UMA purposes.
   MessageCenterStatsCollector stats_collector_;
 
+  // Tracks if shutdown has started.
+  bool is_shutdown_started_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(MessageCenterNotificationManager);
 };