Implement ArcNotificationDelegate::Click() for ArcCustomNotificationItem.

ui/arc/notification/arc_custom_notification_item.cc

Index: ui/arc/notification/arc_custom_notification_item.cc
diff --git a/ui/arc/notification/arc_custom_notification_item.cc b/ui/arc/notification/arc_custom_notification_item.cc
index d3cac1be5ba34295277ec51d626380da325f475e..f44884dd5c039ad80cf19ab073be0df5c6b3e581 100644
--- a/ui/arc/notification/arc_custom_notification_item.cc
+++ b/ui/arc/notification/arc_custom_notification_item.cc
@@ -39,6 +39,8 @@ class ArcNotificationDelegate : public message_center::NotificationDelegate {
 
   void Close(bool by_user) override { item_->Close(by_user); }
 
+  void Click() override { item_->Click(); }
+
  private:
   // The destructor is private since this class is ref-counted.
   ~ArcNotificationDelegate() override {}