arc: Fix custom notification crash on closing

ui/arc/notification/arc_custom_notification_item.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_ARC_NOTIFICATION_ARC_CUSTOM_NOTIFICATION_ITEM_H_
 #define UI_ARC_NOTIFICATION_ARC_CUSTOM_NOTIFICATION_ITEM_H_
 
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "ui/arc/notification/arc_notification_item.h"
 #include "ui/arc/notification/arc_notification_surface_manager.h"
 
 namespace arc {
 
 class ArcCustomNotificationItem
     : public ArcNotificationItem,
       public ArcNotificationSurfaceManager::Observer {
  public:
   class Observer {
    public:
     // Invoked when the notification data for this item has changed.
     virtual void OnItemDestroying() = 0;
 
     // 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;
   };
 
   ArcCustomNotificationItem(ArcNotificationManager* manager,
                             message_center::MessageCenter* message_center,
                             const std::string& notification_key,
                             const AccountId& profile_id);
   ~ArcCustomNotificationItem() override;
 
@@ skipping 14 matching lines @@
 
   bool pinned_ = false;
   base::ObserverList<Observer> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(ArcCustomNotificationItem);
 };
 
 }  // namespace arc
 
 #endif  // UI_ARC_NOTIFICATION_ARC_CUSTOM_NOTIFICATION_ITEM_H_