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

chrome/browser/chromeos/login/supervised/supervised_user_creation_screen.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_SCREEN_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SUPERVISED_SUPERVISED_USER_CREATION_SCREEN_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "chrome/browser/chromeos/camera_presence_notifier.h"
 #include "chrome/browser/chromeos/login/screens/base_screen.h"
 #include "chrome/browser/chromeos/login/supervised/supervised_user_creation_controller.h"
 #include "chrome/browser/image_decoder.h"
 #include "chrome/browser/supervised_user/legacy/supervised_user_sync_service.h"
 #include "chrome/browser/ui/webui/chromeos/login/supervised_user_creation_screen_handler.h"
 #include "chromeos/network/portal_detector/network_portal_detector.h"
 #include "ui/gfx/image/image_skia.h"
 
 class Profile;
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class ErrorScreensHistogramHelper;
 class NetworkState;
 class ScreenManager;
 
 // Class that controls screen showing ui for supervised user creation.
 class SupervisedUserCreationScreen
     : public BaseScreen,
       public SupervisedUserCreationScreenHandler::Delegate,
@@ skipping 30 matching lines @@
 
   // Shows initial screen where managed user name/password are defined and
   // manager is selected.
   void ShowInitialScreen();
 
   // CameraPresenceNotifier::Observer implementation:
   void OnCameraPresenceCheckDone(bool is_camera_present) override;
 
   // SupervisedUserSyncServiceObserver implementation
   void OnSupervisedUserAcknowledged(
-      const std::string& supervised_user_id) override {}
+      const user_manager::UserID& supervised_user_id) override {}
   void OnSupervisedUsersSyncingStopped() override {}
   void OnSupervisedUsersChanged() override;
 
   // BaseScreen implementation:
   void PrepareToShow() override;
   void Show() override;
   void Hide() override;
   std::string GetName() const override;
 
   // SupervisedUserCreationScreenHandler::Delegate implementation:
   void OnActorDestroyed(SupervisedUserCreationScreenHandler* actor) override;
   void CreateSupervisedUser(
       const base::string16& display_name,
       const std::string& supervised_user_password) override;
-  void ImportSupervisedUser(const std::string& user_id) override;
-  void ImportSupervisedUserWithPassword(const std::string& user_id,
+  void ImportSupervisedUser(const user_manager::UserID& user_id) override;
+  void ImportSupervisedUserWithPassword(const user_manager::UserID& user_id,
                                         const std::string& password) override;
-  void AuthenticateManager(const std::string& manager_id,
+  void AuthenticateManager(const user_manager::UserID& manager_id,
                            const std::string& manager_password) override;
   void AbortFlow() override;
   void FinishFlow() override;
   void HideFlow() override;
   bool FindUserByDisplayName(const base::string16& display_name,
                              std::string* out_id) const override;
   void OnPageSelected(const std::string& page) override;
 
   // SupervisedUserController::StatusConsumer overrides.
   void OnCreationError(
@@ skipping 42 matching lines @@
 
   base::WeakPtrFactory<SupervisedUserCreationScreen> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SupervisedUserCreationScreen);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SUPERVISED_SUPERVISED_USER_CREATION_SCREEN_H_