Make GetDisplayedNotifications asynchronous.

content/test/mock_platform_notification_service.h

Index: content/test/mock_platform_notification_service.h
diff --git a/content/test/mock_platform_notification_service.h b/content/test/mock_platform_notification_service.h
index 790053badd1bb331feedb20ccc4bc867f1b0a76e..64496cd960dbe59379abdf6f91df8ae3906b7b99 100644
--- a/content/test/mock_platform_notification_service.h
+++ b/content/test/mock_platform_notification_service.h
@@ -71,9 +71,9 @@ class MockPlatformNotificationService : public PlatformNotificationService {
       const NotificationResources& notification_resources) override;
   void ClosePersistentNotification(BrowserContext* browser_context,
                                    const std::string& notification_id) override;
-  bool GetDisplayedNotifications(
+  void GetDisplayedNotifications(
       BrowserContext* browser_context,
-      std::set<std::string>* displayed_notifications) override;
+      const DisplayedNotificationsCallback& callback) override;
 
  protected:
   // Checks if |origin| has permission to display notifications. May be called