Make GetDisplayedNotifications asynchronous.

chrome/browser/notifications/message_center_display_service.h

Index: chrome/browser/notifications/message_center_display_service.h
diff --git a/chrome/browser/notifications/message_center_display_service.h b/chrome/browser/notifications/message_center_display_service.h
index 05a8fc377d6344c0d2f4a9326a968fe611e8fea1..2dcf0e9c3a797ef617ec08d2835ec5acac059aa9 100644
--- a/chrome/browser/notifications/message_center_display_service.h
+++ b/chrome/browser/notifications/message_center_display_service.h
@@ -27,7 +27,8 @@ class MessageCenterDisplayService : 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 DisplayedNotificationsCallback& callback) const override;
 
  private:
   Profile* profile_;