Implement TOGGLE_EXPANSION action for the ARC notifications.

ui/message_center/views/custom_notification_view.h

 // 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.
 
 #ifndef UI_MESSAGE_CENTER_VIEWS_CUSTOM_NOTIFICATION_VIEW_H_
 #define UI_MESSAGE_CENTER_VIEWS_CUSTOM_NOTIFICATION_VIEW_H_
 
 #include "base/macros.h"
 #include "ui/message_center/message_center_export.h"
 #include "ui/message_center/views/message_view.h"
@@ skipping 28 matching lines @@
 
   // Overridden from views::View:
   const char* GetClassName() const override;
   gfx::Size GetPreferredSize() const override;
   void Layout() override;
   bool HasFocus() const override;
   void RequestFocus() override;
   void OnPaint(gfx::Canvas* canvas) override;
   bool OnKeyPressed(const ui::KeyEvent& event) override;
   void ChildPreferredSizeChanged(View* child) override;
+  bool OnMousePressed(const ui::MouseEvent& event) override;
 
  private:
   friend class CustomNotificationViewTest;
 
   // The view for the custom content. Owned by view hierarchy.
   views::View* contents_view_ = nullptr;
   std::unique_ptr<CustomNotificationContentViewDelegate>
       contents_view_delegate_;
 
   std::unique_ptr<views::Painter> focus_painter_;
 
   DISALLOW_COPY_AND_ASSIGN(CustomNotificationView);
 };
 
 }  // namespace message_center
 
 #endif  // UI_MESSAGE_CENTER_VIEWS_CUSTOM_NOTIFICATION_VIEW_H_