arc: Fix UpdatePinnedState crash

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 f0ba730697cd75b070bb17a7d42038359d2cd267..b38cd323645366fcd115931010cfa5a325e517d9 100644
--- a/ui/arc/notification/arc_custom_notification_view.cc
+++ b/ui/arc/notification/arc_custom_notification_view.cc
@@ -230,7 +230,8 @@ void ArcCustomNotificationView::SetSurface(exo::NotificationSurface* surface) {
     if (GetWidget())
       AttachSurface();
 
-    UpdatePinnedState();
+    if (item_)

[yoshiki, 2016/09/20 03:43:06]

The patch itself lgtm, but why item is null here?

It means surface is changed after OnItemDestroying, but isn't it weird?

[xiyuan, 2016/09/20 15:58:16]

There is a racing between ArcNotificationService and wayland on notification
close. |item_| is set to null when a notification is removed and mojo call is
made. However, the window closing is async.  Even though we close the window
before we do mojo call in ArcNotificationService. It is still possible that the
notification surface could exist and update via wayland for a short while. Hence
the crash.

+      UpdatePinnedState();
   }
 }
 
@@ -268,6 +269,8 @@ void ArcCustomNotificationView::UpdateCloseButtonVisiblity() {
 }
 
 void ArcCustomNotificationView::UpdatePinnedState() {
+  DCHECK(item_);
+
   if (item_->pinned() && floating_close_button_widget_) {
     floating_close_button_widget_.reset();
   } else if (!item_->pinned() && !floating_close_button_widget_) {