Extract the view of control buttons on notification into a separated class

ui/arc/notification/arc_notification_delegate.h

Index: ui/arc/notification/arc_notification_delegate.h
diff --git a/ui/arc/notification/arc_notification_delegate.h b/ui/arc/notification/arc_notification_delegate.h
index d9922738d88f38ff6158d8f258889dfde43b0153..9241fcec2ee7e324005178730a4c1310065f0a0b 100644
--- a/ui/arc/notification/arc_notification_delegate.h
+++ b/ui/arc/notification/arc_notification_delegate.h
@@ -32,6 +32,7 @@ class ArcNotificationDelegate : public message_center::NotificationDelegate {
       const message_center::Notification& notification) override;
   void Close(bool by_user) override;
   void Click() override;
+  bool SettingsClick() override;
 
  private:
   // The destructor is private since this class is ref-counted.