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

chromeos/login/auth/stub_authenticator.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 CHROMEOS_LOGIN_AUTH_STUB_AUTHENTICATOR_H_
 #define CHROMEOS_LOGIN_AUTH_STUB_AUTHENTICATOR_H_
 
 #include <string>
 
 #include "base/message_loop/message_loop_proxy.h"
 #include "chromeos/chromeos_export.h"
 #include "chromeos/login/auth/authenticator.h"
 #include "chromeos/login/auth/user_context.h"
 
 namespace content {
 class BrowserContext;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class AuthStatusConsumer;
 
 class CHROMEOS_EXPORT StubAuthenticator : public Authenticator {
  public:
   StubAuthenticator(AuthStatusConsumer* consumer,
                     const UserContext& expected_user_context);
 
   // Authenticator:
   void CompleteLogin(content::BrowserContext* context,
                      const UserContext& user_context) override;
   void AuthenticateToLogin(content::BrowserContext* context,
                            const UserContext& user_context) override;
   void AuthenticateToUnlock(const UserContext& user_context) override;
   void LoginAsSupervisedUser(const UserContext& user_context) override;
   void LoginOffTheRecord() override;
   void LoginAsPublicSession(const UserContext& user_context) override;
-  void LoginAsKioskAccount(const std::string& app_user_id,
+  void LoginAsKioskAccount(const user_manager::UserID& app_user_id,
                            bool use_guest_mount) override;
   void OnAuthSuccess() override;
   void OnAuthFailure(const AuthFailure& failure) override;
   void RecoverEncryptedData(const std::string& old_password) override;
   void ResyncEncryptedData() override;
 
   virtual void SetExpectedCredentials(const UserContext& user_context);
 
  protected:
   ~StubAuthenticator() override;
 
  private:
   UserContext expected_user_context_;
   scoped_refptr<base::MessageLoopProxy> message_loop_;
 
   DISALLOW_COPY_AND_ASSIGN(StubAuthenticator);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_LOGIN_AUTH_STUB_AUTHENTICATOR_H_