Implement TOGGLE_EXPANSION action for the ARC notifications.

ui/message_center/views/custom_notification_view.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/message_center/views/custom_notification_view.h"
 
 #include <algorithm>
 
 #include "ui/base/ime/input_method.h"
 #include "ui/base/ime/text_input_client.h"
@@ skipping 138 matching lines @@
   return MessageView::OnKeyPressed(event);
 }
 
 void CustomNotificationView::ChildPreferredSizeChanged(View* child) {
   // Notify MessageCenterController when the custom content changes size,
   // as it may need to relayout.
   if (controller())
     controller()->UpdateNotificationSize(notification_id());
 }
 
+bool CustomNotificationView::OnMousePressed(const ui::MouseEvent& event) {
+  // TODO(yhanada): This hack will be removed when HandleAccessibleAction will
+  // be used for handling click events.
+  if (contents_view_ && contents_view_->OnMousePressed(event))
+    return true;
+  return MessageView::OnMousePressed(event);
+}
+
 }  // namespace message_center