arc: Fix custom notification crash on closing

ui/arc/notification/arc_custom_notification_item.h

Index: ui/arc/notification/arc_custom_notification_item.h
diff --git a/ui/arc/notification/arc_custom_notification_item.h b/ui/arc/notification/arc_custom_notification_item.h
index 5bc55b916ef33e2da1ff3195a7cd516f55011288..14c9446a7b0c9186a2b3671d92841dadcf917202 100644
--- a/ui/arc/notification/arc_custom_notification_item.h
+++ b/ui/arc/notification/arc_custom_notification_item.h
@@ -24,6 +24,9 @@ class ArcCustomNotificationItem
     // Invoked when the pinned stated is changed.
     virtual void OnItemPinnedChanged() = 0;
 
+    // Invoked when the notification surface for this item is gone.
+    virtual void OnItemNotificationSurfaceRemoved() = 0;
+
    protected:
     virtual ~Observer() = default;
   };