Make GetDisplayedNotifications asynchronous.

chrome/browser/notifications/native_notification_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_NATIVE_NOTIFICATION_DISPLAY_SERVICE_H_
 #define CHROME_BROWSER_NOTIFICATIONS_NATIVE_NOTIFICATION_DISPLAY_SERVICE_H_
 
 #include <map>
 #include <memory>
 #include <set>
@@ skipping 20 matching lines @@
       Profile* profile,
       NotificationPlatformBridge* notification_bridge);
   ~NativeNotificationDisplayService() 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;
 
   // Used by the notification bridge to propagate back events (click, close...).
   void ProcessNotificationOperation(NotificationCommon::Operation operation,
                                     NotificationCommon::Type notification_type,
                                     const std::string& origin,
                                     const std::string& notification_id,
                                     int action_index,
                                     const base::NullableString16& reply);
 
   // Registers an implementation object to handle notification operations
@@ skipping 10 matching lines @@
 
   Profile* profile_;
   NotificationPlatformBridge* notification_bridge_;
   std::map<NotificationCommon::Type, std::unique_ptr<NotificationHandler>>
       notification_handlers_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeNotificationDisplayService);
 };
 
 #endif  // CHROME_BROWSER_NOTIFICATIONS_NATIVE_NOTIFICATION_DISPLAY_SERVICE_H_