Make GetDisplayedNotifications asynchronous.

chrome/browser/notifications/platform_notification_service_impl.h

Index: chrome/browser/notifications/platform_notification_service_impl.h
diff --git a/chrome/browser/notifications/platform_notification_service_impl.h b/chrome/browser/notifications/platform_notification_service_impl.h
index 12c6e6181d4190459646c7b7af238fdfbf00576a..d497d81c010705002722989b1365aa7ddf18c80f 100644
--- a/chrome/browser/notifications/platform_notification_service_impl.h
+++ b/chrome/browser/notifications/platform_notification_service_impl.h
@@ -95,9 +95,9 @@ class PlatformNotificationServiceImpl
       const content::NotificationResources& notification_resources) override;
   void ClosePersistentNotification(content::BrowserContext* browser_context,
                                    const std::string& notification_id) override;
-  bool GetDisplayedNotifications(
+  void GetDisplayedNotifications(
       content::BrowserContext* browser_context,
-      std::set<std::string>* displayed_notifications) override;
+      const DisplayedNotificationsCallback& callback) override;
 
  private:
   friend struct base::DefaultSingletonTraits<PlatformNotificationServiceImpl>;