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

chrome/browser/ui/webui/chromeos/login/supervised_user_creation_screen_handler.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_UI_WEBUI_CHROMEOS_LOGIN_SUPERVISED_USER_CREATION_SCREEN_HANDLER_H_
 #define CHROME_BROWSER_UI_WEBUI_CHROMEOS_LOGIN_SUPERVISED_USER_CREATION_SCREEN_HANDLER_H_
 
 #include <string>
 
 #include "base/compiler_specific.h"
 #include "base/strings/string16.h"
 #include "chrome/browser/ui/webui/chromeos/login/base_screen_handler.h"
 #include "components/user_manager/user_image/default_user_images.h"
 #include "content/public/browser/web_ui.h"
 
 namespace base {
 class ListValue;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class SupervisedUserCreationScreenHandler : public BaseScreenHandler {
  public:
   class Delegate {
    public:
     virtual ~Delegate() {}
 
     // This method is called, when actor is being destroyed. Note, if Delegate
     // is destroyed earlier then it has to call SetDelegate(NULL).
     virtual void OnActorDestroyed(
         SupervisedUserCreationScreenHandler* actor) = 0;
 
     // Starts supervised user creation flow, with manager identified by
     // |manager_id| and |manager_password|.
-    virtual void AuthenticateManager(const std::string& manager_id,
+    virtual void AuthenticateManager(const user_manager::UserID& manager_id,
                                      const std::string& manager_password) = 0;
 
     // Starts supervised user creation flow, with supervised user that would
     // have |display_name| and authenticated by the |supervised_user_password|.
     virtual void CreateSupervisedUser(
         const base::string16& display_name,
         const std::string& supervised_user_password) = 0;
 
     // Look up if user with name |display_name| already exist and can be
     // imported. Returns user ID in |out_id|. Returns true if user was found,
     // false otherwise.
     virtual bool FindUserByDisplayName(const base::string16& display_name,
                                        std::string *out_id) const = 0;
 
     // Starts supervised user import flow for user identified with |user_id|.
-    virtual void ImportSupervisedUser(const std::string& user_id) = 0;
+    virtual void ImportSupervisedUser(const user_manager::UserID& user_id) = 0;
     // Starts supervised user import flow for user identified with |user_id| and
     // additional |password|.
     virtual void ImportSupervisedUserWithPassword(
-        const std::string& user_id, const std::string& password) = 0;
+        const user_manager::UserID& user_id, const std::string& password) = 0;
 
     virtual void AbortFlow() = 0;
     virtual void FinishFlow() = 0;
     virtual void HideFlow() = 0;
 
     virtual void OnPhotoTaken(const std::string& raw_data) = 0;
     virtual void OnImageSelected(const std::string& image_url,
                                  const std::string& image_type) = 0;
     virtual void OnImageAccepted() = 0;
     virtual void OnPageSelected(const std::string& page) = 0;
@@ skipping 67 matching lines @@
   void UpdateText(const std::string& element_id, const base::string16& text);
 
   Delegate* delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(SupervisedUserCreationScreenHandler);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_UI_WEBUI_CHROMEOS_LOGIN_SUPERVISED_USER_CREATION_SCREEN_HANDLER_H_