arc: Fix UpdatePinnedState crash

Transient notifications get into a situation that the item is destroyed
via mojo before its surface gets mapped via wayland. Check |item_|
before UpdatePinnedState() call in SetSurface to fix the crash.

BUG=647829
TBR=yoshiki@chromium.org

Review-Url: https://codereview.chromium.org/2345373002
Cr-Commit-Position: refs/heads/master@{#419508}

ui/arc/notification/arc_custom_notification_view.cc

@@ -230,7 +230,8 @@ void ArcCustomNotificationView::SetSurface(exo::NotificationSurface* surface) {
     if (GetWidget())
       AttachSurface();
 
-    UpdatePinnedState();
+    if (item_)
+      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_) {