We should use UserID object to identify users instead of username.

chrome/browser/supervised_user/legacy/supervised_user_sync_service_observer.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_SUPERVISED_USER_LEGACY_SUPERVISED_USER_SYNC_SERVICE_OBSERVER_H_
 #define CHROME_BROWSER_SUPERVISED_USER_LEGACY_SUPERVISED_USER_SYNC_SERVICE_OBSERVER_H_
 
+namespace user_manager {
+class UserID;
+}
+
 class SupervisedUserSyncServiceObserver {
  public:
   // Called when the Sync server has acknowledged a newly
   // created supervised user.
   virtual void OnSupervisedUserAcknowledged(
-      const std::string& supervised_user_id) = 0;
+      const user_manager::UserID& supervised_user_id) = 0;
 
   virtual void OnSupervisedUsersSyncingStopped() = 0;
 
   virtual void OnSupervisedUsersChanged() = 0;
 
  protected:
   virtual ~SupervisedUserSyncServiceObserver() {}
 };
 
 #endif  // CHROME_BROWSER_SUPERVISED_USER_LEGACY_SUPERVISED_USER_SYNC_SERVICE_OBSERVER_H_