arc: Fix custom notification crash on closing

ui/arc/notification/arc_custom_notification_view.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_VIEW_H_
 #define UI_ARC_NOTIFICATION_ARC_CUSTOM_NOTIFICATION_VIEW_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 28 matching lines @@
   void ViewHierarchyChanged(
       const ViewHierarchyChangedDetails& details) override;
   void Layout() override;
 
   // views::ButtonListener
   void ButtonPressed(views::Button* sender, const ui::Event& event) override;
 
   // ArcCustomNotificationItem::Observer
   void OnItemDestroying() override;
   void OnItemPinnedChanged() override;
+  void OnItemNotificationSurfaceRemoved() override;
 
   ArcCustomNotificationItem* item_;
-  exo::NotificationSurface* const surface_;
+  exo::NotificationSurface* surface_;
 
   // A close button on top of NotificationSurface. Needed because the
   // aura::Window of NotificationSurface is added after hosting widget's
   // RootView thus standard notification close button is always below
   // it.
   std::unique_ptr<views::Widget> floating_close_button_widget_;
 
   views::ImageButton* floating_close_button_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(ArcCustomNotificationView);
 };
 
 }  // namespace arc
 
 #endif  // UI_ARC_NOTIFICATION_ARC_CUSTOM_NOTIFICATION_VIEW_H_