Make GetDisplayedNotifications asynchronous.

chrome/browser/notifications/notification_platform_bridge_mac.h

Index: chrome/browser/notifications/notification_platform_bridge_mac.h
diff --git a/chrome/browser/notifications/notification_platform_bridge_mac.h b/chrome/browser/notifications/notification_platform_bridge_mac.h
index 43ab69fb877b9b43d8bfb713442fe50b7a966044..4a2f6b3c41bd1f6e50f102d4c1fd3680349a05e3 100644
--- a/chrome/browser/notifications/notification_platform_bridge_mac.h
+++ b/chrome/browser/notifications/notification_platform_bridge_mac.h
@@ -39,9 +39,10 @@ class NotificationPlatformBridgeMac : public NotificationPlatformBridge {
 
   void Close(const std::string& profile_id,
              const std::string& notification_id) override;
-  bool GetDisplayed(const std::string& profile_id,
-                    bool incognito,
-                    std::set<std::string>* notifications) const override;
+  void GetDisplayed(
+      const std::string& profile_id,
+      bool incognito,
+      const DisplayedNotificationsCallback& callback) const override;
 
   // Processes a notification response generated from a user action
   // (click close, etc.).