Add a floating close button for arc custom notification

ui/arc/notification/arc_notification_item.h

Index: ui/arc/notification/arc_notification_item.h
diff --git a/ui/arc/notification/arc_notification_item.h b/ui/arc/notification/arc_notification_item.h
index bdaf1d51c2c8a6d86d0e0edd8acf2f895c145815..11977697b043fc08d9fc9f290cc2fb8028425e7a 100644
--- a/ui/arc/notification/arc_notification_item.h
+++ b/ui/arc/notification/arc_notification_item.h
@@ -59,6 +59,7 @@ class ArcNotificationItem {
   const AccountId& profile_id() const { return profile_id_; }
   const std::string& notification_key() const { return notification_key_; }
   const std::string& notification_id() const { return notification_id_; }
+  message_center::MessageCenter* message_center() { return message_center_; }
 
   message_center::Notification* pending_notification() {
     return notification_.get();