arc: Fix crash on close a notification

ui/arc/notification/arc_custom_notification_view.cc

Index: ui/arc/notification/arc_custom_notification_view.cc
diff --git a/ui/arc/notification/arc_custom_notification_view.cc b/ui/arc/notification/arc_custom_notification_view.cc
index d9da938f33ca604aa89fdbb3bb5b76a8c647a7a9..f0ba730697cd75b070bb17a7d42038359d2cd267 100644
--- a/ui/arc/notification/arc_custom_notification_view.cc
+++ b/ui/arc/notification/arc_custom_notification_view.cc
@@ -213,6 +213,9 @@ 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());