Make GetDisplayedNotifications asynchronous.

chrome/browser/notifications/native_notification_display_service.h

Index: chrome/browser/notifications/native_notification_display_service.h
diff --git a/chrome/browser/notifications/native_notification_display_service.h b/chrome/browser/notifications/native_notification_display_service.h
index e0240fe3357709a2398e566a57e4c16699380fa8..3b29b15e5d70609789615c574f0393bfcdbd7527 100644
--- a/chrome/browser/notifications/native_notification_display_service.h
+++ b/chrome/browser/notifications/native_notification_display_service.h
@@ -38,7 +38,7 @@ class NativeNotificationDisplayService : public NotificationDisplayService {
                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 NotificationResultCallback& callback) const override;
 
   // Used by the notification bridge to propagate back events (click, close...).
   void ProcessNotificationOperation(NotificationCommon::Operation operation,