arc: Fix crash on close a notification

ui/arc/notification/arc_custom_notification_view.cc

 // 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.
 
 #include "ui/arc/notification/arc_custom_notification_view.h"
 
 #include "base/auto_reset.h"
 #include "components/exo/notification_surface.h"
 #include "components/exo/surface.h"
 #include "third_party/skia/include/core/SkColor.h"
@@ skipping 195 matching lines @@
   floating_close_button_widget_->Init(params);
   floating_close_button_widget_->SetContentsView(floating_close_button_);
 
   Layout();
 }
 
 void ArcCustomNotificationView::SetSurface(exo::NotificationSurface* surface) {
   if (surface_ == surface)
     return;
 
+  // Reset |floating_close_button_widget_| when |surface_| is changed.
+  floating_close_button_widget_.reset();
+
   if (surface_ && surface_->window()) {
     surface_->window()->RemoveObserver(this);
     surface_->window()->RemovePreTargetHandler(event_forwarder_.get());
   }
 
   surface_ = surface;
 
   if (surface_ && surface_->window()) {
     surface_->window()->AddObserver(this);
     surface_->window()->AddPreTargetHandler(event_forwarder_.get());
@@ skipping 203 matching lines @@
 
 void ArcCustomNotificationView::OnNotificationSurfaceRemoved(
     exo::NotificationSurface* surface) {
   if (surface->notification_id() != notification_key_)
     return;
 
   SetSurface(nullptr);
 }
 
 }  // namespace arc