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

chrome/browser/chromeos/login/supervised/supervised_user_creation_controller.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_CHROMEOS_LOGIN_SUPERVISED_SUPERVISED_USER_CREATION_CONTROLLER_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SUPERVISED_SUPERVISED_USER_CREATION_CONTROLLER_H_
 
 #include <string>
 
 #include "base/files/file_path.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "base/strings/string16.h"
 #include "base/values.h"
 #include "chrome/browser/chromeos/login/supervised/supervised_user_authenticator.h"
 #include "chrome/browser/supervised_user/legacy/supervised_user_registration_utility.h"
 
 class Profile;
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class SupervisedUserCreationController {
  public:
   // This constant is used to indicate that user does not have one of default
   // avatars: either he has no chromeos avatar at all, or has an external
   // image as an avatar.
   static const int kDummyAvatarIndex;
 
   enum ErrorCode {
@@ skipping 52 matching lines @@
                            const std::string& sync_id,
                            const std::string& master_key,
                            const base::DictionaryValue* password_data,
                            const std::string& encryption_key,
                            const std::string& signature_key) = 0;
 
   virtual void SetManagerProfile(Profile* manager_profile) = 0;
   virtual Profile* GetManagerProfile() = 0;
   virtual void CancelCreation() = 0;
   virtual void FinishCreation() = 0;
-  virtual std::string GetSupervisedUserId() = 0;
+  virtual const user_manager::UserID& GetSupervisedUserId() const = 0;
 
  protected:
   // Pointer to the current instance of the controller to be used by
   // automation tests.
   static SupervisedUserCreationController* current_controller_;
 
   StatusConsumer* consumer_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(SupervisedUserCreationController);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SUPERVISED_SUPERVISED_USER_CREATION_CONTROLLER_H_