Make GetDisplayedNotifications asynchronous.

chrome/browser/notifications/message_center_display_service.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 CHROME_BROWSER_NOTIFICATIONS_MESSAGE_CENTER_DISPLAY_SERVICE_H_
 #define CHROME_BROWSER_NOTIFICATIONS_MESSAGE_CENTER_DISPLAY_SERVICE_H_
 
 #include "base/macros.h"
 #include "chrome/browser/notifications/notification_common.h"
 #include "chrome/browser/notifications/notification_display_service.h"
 
 class Notification;
 class NotificationUIManager;
 class Profile;
 
 // Implementation of display service for notifications displayed by chrome
 // instead of the native platform notification center.
 class MessageCenterDisplayService : public NotificationDisplayService {
  public:
   MessageCenterDisplayService(Profile* profile,
                               NotificationUIManager* ui_manager);
   ~MessageCenterDisplayService() override;
 
   // NotificationDisplayService implementation.
   void Display(NotificationCommon::Type notification_type,
                const std::string& notification_id,
                const Notification& notification) override;
   void Close(NotificationCommon::Type notification_type,
              const std::string& notification_id) override;
-  bool GetDisplayed(std::set<std::string>* notifications) const override;
+  void GetDisplayed(
+      const DisplayedNotificationsCallback& callback) const override;
 
  private:
   Profile* profile_;
   NotificationUIManager* ui_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageCenterDisplayService);
 };
 
 #endif  // CHROME_BROWSER_NOTIFICATIONS_MESSAGE_CENTER_DISPLAY_SERVICE_H_