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

chromeos/login/auth/cryptohome_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_CRYPTOHOME_AUTHENTICATOR_H_
 #define CHROMEOS_LOGIN_AUTH_CRYPTOHOME_AUTHENTICATOR_H_
 
 #include <string>
 
 #include "base/compiler_specific.h"
@@ skipping 116 matching lines @@
   // Initiates login into a public session.
   // Mounts an ephemeral cryptohome and notifies consumer on the
   // success/failure.
   void LoginAsPublicSession(const UserContext& user_context) override;
 
   // Initiates login into the kiosk mode account identified by |app_user_id|.
   // Mounts an ephemeral guest cryptohome if |use_guest_mount| is |true|.
   // Otherwise, mounts a public cryptohome, which will be ephemeral if the
   // |DeviceEphemeralUsersEnabled| policy is enabled and non-ephemeral
   // otherwise.
-  void LoginAsKioskAccount(const std::string& app_user_id,
+  void LoginAsKioskAccount(const user_manager::UserID& app_user_id,
                            bool use_guest_mount) override;
 
   // These methods must be called on the UI thread, as they make DBus calls
   // and also call back to the login UI.
   void OnAuthSuccess() override;
   void OnAuthFailure(const AuthFailure& error) override;
   void RecoverEncryptedData(const std::string& old_password) override;
   void ResyncEncryptedData() override;
 
   // AuthAttemptStateResolver overrides.
@@ skipping 109 matching lines @@
   // When |remove_user_data_on_failure_| is set, we delay calling
   // consumer_->OnAuthFailure() until we removed the user cryptohome.
   const AuthFailure* delayed_login_failure_;
 
   DISALLOW_COPY_AND_ASSIGN(CryptohomeAuthenticator);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_LOGIN_AUTH_CRYPTOHOME_AUTHENTICATOR_H_