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

chromeos/login/auth/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_AUTHENTICATOR_H_
 #define CHROMEOS_LOGIN_AUTH_AUTHENTICATOR_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/memory/ref_counted.h"
 #include "chromeos/chromeos_export.h"
 #include "chromeos/login/auth/auth_status_consumer.h"
 #include "google_apis/gaia/gaia_auth_consumer.h"
 
 namespace content {
 class BrowserContext;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class UserContext;
 
 // An interface for objects that will authenticate a Chromium OS user.
 // Callbacks will be called on the UI thread:
 // 1. On successful authentication, will call consumer_->OnAuthSuccess().
 // 2. On failure, will call consumer_->OnAuthFailure().
 // 3. On password change, will call consumer_->OnPasswordChangeDetected().
 class CHROMEOS_EXPORT Authenticator
@@ skipping 23 matching lines @@
   // Initiates incognito ("browse without signing in") login.
   virtual void LoginOffTheRecord() = 0;
 
   // Initiates login into the public account identified by |user_context|.
   virtual void LoginAsPublicSession(const UserContext& user_context) = 0;
 
   // Initiates login into kiosk mode account identified by |app_user_id|.
   // The |app_user_id| is a generated username for the account.
   // |use_guest_mount| specifies whether to force the session to use a
   // guest mount. If this is false, we use mount a public cryptohome.
-  virtual void LoginAsKioskAccount(const std::string& app_user_id,
+  virtual void LoginAsKioskAccount(const user_manager::UserID& app_user_id,
                                    bool use_guest_mount) = 0;
 
   // Notifies caller that login was successful. Must be called on the UI thread.
   virtual void OnAuthSuccess() = 0;
 
   // Must be called on the UI thread.
   virtual void OnAuthFailure(const AuthFailure& error) = 0;
 
   // Call these methods on the UI thread.
   // If a password logs the user in online, but cannot be used to
@@ skipping 25 matching lines @@
 
  private:
   friend class base::RefCountedThreadSafe<Authenticator>;
 
   DISALLOW_COPY_AND_ASSIGN(Authenticator);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_LOGIN_AUTH_AUTHENTICATOR_H_