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

chrome/browser/chromeos/app_mode/kiosk_profile_loader.h

 // Copyright 2013 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_APP_MODE_KIOSK_PROFILE_LOADER_H_
 #define CHROME_BROWSER_CHROMEOS_APP_MODE_KIOSK_PROFILE_LOADER_H_
 
 #include <string>
 
 #include "base/basictypes.h"
@@ skipping 15 matching lines @@
  public:
   class Delegate {
    public:
     virtual void OnProfileLoaded(Profile* profile) = 0;
     virtual void OnProfileLoadFailed(KioskAppLaunchError::Error error) = 0;
 
    protected:
     virtual ~Delegate() {}
   };
 
-  KioskProfileLoader(const std::string& app_user_id,
+  KioskProfileLoader(const user_manager::UserID& app_user_id,
                      bool use_guest_mount,
                      Delegate* delegate);
 
   ~KioskProfileLoader() override;
 
   // Starts profile load. Calls delegate on success or failure.
   void Start();
 
  private:
   class CryptohomedChecker;
 
   void LoginAsKioskAccount();
   void ReportLaunchResult(KioskAppLaunchError::Error error);
 
   // LoginPerformer::Delegate overrides:
   void OnAuthSuccess(const UserContext& user_context) override;
   void OnAuthFailure(const AuthFailure& error) override;
-  void WhiteListCheckFailed(const std::string& email) override;
+  void WhiteListCheckFailed(const user_manager::UserID& user_id) override;
   void PolicyLoadFailed() override;
-  void OnOnlineChecked(const std::string& email, bool success) override;
+  void OnOnlineChecked(const user_manager::UserID& user_id, bool success) override;
 
   // UserSessionManagerDelegate implementation:
   void OnProfilePrepared(Profile* profile, bool browser_launched) override;
 
-  std::string user_id_;
+  user_manager::UserID user_id_;
   bool use_guest_mount_;
   Delegate* delegate_;
   scoped_ptr<CryptohomedChecker> cryptohomed_checker_;
   scoped_ptr<LoginPerformer> login_performer_;
 
   DISALLOW_COPY_AND_ASSIGN(KioskProfileLoader);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_APP_MODE_KIOSK_PROFILE_LOADER_H_