message: Allow permission results to register proper notification blocks
diff: @@ -110,6 +110,12 @@ public:
  return m_results.async(std::move(target));
  }
  + // Add a notification callback to this Results.
  + NotificationToken add_notification_callback(CollectionChangeCallback cb) &
  + {
  + return m_results.add_notification_callback(std::move(cb));
  + }
  +
  // Create a new instance by further filtering this instance.
  PermissionResults filter(Query&& q) const
  {
  