Make GetDisplayedNotifications asynchronous.

content/public/browser/platform_notification_service.h

Index: content/public/browser/platform_notification_service.h
diff --git a/content/public/browser/platform_notification_service.h b/content/public/browser/platform_notification_service.h
index c1ad9ecd12d03f138b9723274bd68626e4583cad..7689ff65699bc9a352cb49dafb2c792460910a17 100644
--- a/content/public/browser/platform_notification_service.h
+++ b/content/public/browser/platform_notification_service.h
@@ -33,6 +33,10 @@ class CONTENT_EXPORT PlatformNotificationService {
  public:
   virtual ~PlatformNotificationService() {}
 
+  using DisplayedNotificationsCallback =
+      base::Callback<void(std::unique_ptr<std::set<std::string>>,
+                          bool /* supports synchronization */)>;
+
   // Checks if |origin| has permission to display Web Notifications.
   // This method must only be called on the UI thread.
   virtual blink::mojom::PermissionStatus CheckPermissionOnUIThread(
@@ -78,12 +82,11 @@ class CONTENT_EXPORT PlatformNotificationService {
       BrowserContext* browser_context,
       const std::string& notification_id) = 0;
 
-  // Writes the ids of all currently displaying notifications for the
-  // given |browser_context| to |displayed_notifications|. Returns whether the
-  // platform is able to provide such a set.
-  virtual bool GetDisplayedNotifications(
+  // Retrieves the ids of all currently displaying notifications and
+  // posts |callback| with the result.
+  virtual void GetDisplayedNotifications(
       BrowserContext* browser_context,
-      std::set<std::string>* displayed_notifications) = 0;
+      const DisplayedNotificationsCallback& callback) = 0;
 };
 
 }  // namespace content