Add a floating close button for arc custom notification

ui/arc/notification/arc_notification_item.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_ARC_NOTIFICATION_ARC_NOTIFICATION_ITEM_H_
 #define UI_ARC_NOTIFICATION_ARC_NOTIFICATION_ITEM_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 41 matching lines @@
 
   // Add |notification_| to message center and update again if there is
   // |newer_data_|.
   void AddToMessageCenter();
 
   bool CalledOnValidThread() const;
 
   const AccountId& profile_id() const { return profile_id_; }
   const std::string& notification_key() const { return notification_key_; }
   const std::string& notification_id() const { return notification_id_; }
+  message_center::MessageCenter* message_center() { return message_center_; }
 
   message_center::Notification* pending_notification() {
     return notification_.get();
   }
 
  private:
   void OnImageDecoded(const SkBitmap& bitmap);
 
   ArcNotificationManager* const manager_;
   message_center::MessageCenter* const message_center_;
@@ skipping 20 matching lines @@
 
   base::ThreadChecker thread_checker_;
   base::WeakPtrFactory<ArcNotificationItem> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ArcNotificationItem);
 };
 
 }  // namespace arc
 
 #endif  // UI_ARC_NOTIFICATION_ARC_NOTIFICATION_ITEM_H_