Transfer responsibility for providing a close button for a notification to each implementation of M…

ui/message_center/notification_delegate_views.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/notification_delegate.h"
 
 #include "ui/views/view.h"
 
 namespace message_center {
 
-std::unique_ptr<views::View> NotificationDelegate::CreateCustomContent() {
-  return std::unique_ptr<views::View>();
+std::tuple<
+    std::unique_ptr<views::View>,
+    std::unique_ptr<message_center::CustomNotificationContentViewDelegate>>
+NotificationDelegate::CreateCustomContent() {
+  return std::make_tuple(nullptr, nullptr);
 }
 
 }  // namespace message_center