arc: Fix crash on close a notification

Speculative fix of the notification close crash that the underlying
notification surface is changed after the floating close button
is created.

BUG=646634

Review-Url: https://codereview.chromium.org/2338423002
Cr-Commit-Position: refs/heads/master@{#418881}

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());