[Notifications] Fix swipe to close for ARC notifications.

ui/message_center/views/custom_notification_content_view_delegate.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_CUSTOM_NOTIFICATION_CONTENT_VIEW_DELEGATE_H_
 #define UI_MESSAGE_CENTER_CUSTOM_NOTIFICATION_CONTENT_VIEW_DELEGATE_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "ui/message_center/message_center_export.h"
 
 namespace views {
 class View;
 }  // namespace views
 
 namespace message_center {
 
 // Delegate for a view that is hosted by CustomNotificationView.
 class MESSAGE_CENTER_EXPORT CustomNotificationContentViewDelegate {
  public:
   virtual bool IsCloseButtonFocused() const = 0;
   virtual void RequestFocusOnCloseButton() = 0;
   virtual bool IsPinned() const = 0;
   virtual void UpdateControlButtonsVisibility() = 0;
+  virtual void OnSlideChanged() = 0;
 };
 
 // The struct to hold a view and CustomNotificationContentViewDelegate
 // associating with the view.
 struct MESSAGE_CENTER_EXPORT CustomContent {
  public:
   CustomContent(
       std::unique_ptr<views::View> view,
       std::unique_ptr<CustomNotificationContentViewDelegate> delegate);
   ~CustomContent();
 
   std::unique_ptr<views::View> view;
   std::unique_ptr<CustomNotificationContentViewDelegate> delegate;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(CustomContent);
 };
 
 }  // namespace message_center
 
 #endif  // UI_MESSAGE_CENTER_CUSTOM_NOTIFICATION_CONTENT_VIEW_DELEGATE_H_