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

chrome/browser/chromeos/login/lock/screen_locker.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_CHROMEOS_LOGIN_LOCK_SCREEN_LOCKER_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_LOCK_SCREEN_LOCKER_H_
 
 #include <string>
 
 #include "base/callback_forward.h"
@@ skipping 123 matching lines @@
 
   // Sets the authenticator.
   void SetAuthenticator(Authenticator* authenticator);
 
   // Called when the screen lock is ready.
   void ScreenLockReady();
 
   // Called when screen locker is safe to delete.
   static void ScheduleDeletion();
 
-  // Returns true if |username| is found among logged in users.
-  bool IsUserLoggedIn(const std::string& username);
+  // Returns true if |user_id| is found among logged in users.
+  bool IsUserLoggedIn(const user_manager::UserID& user_id);
 
   // Looks up user in unlock user list.
-  const user_manager::User* FindUnlockUser(const std::string& user_id);
+  const user_manager::User* FindUnlockUser(const user_manager::UserID& user_id);
 
   // ScreenLockerDelegate instance in use.
   scoped_ptr<ScreenLockerDelegate> delegate_;
 
   // Users that can unlock the device.
   user_manager::UserList users_;
 
   // Used to authenticate the user to unlock.
   scoped_refptr<Authenticator> authenticator_;
 
@@ skipping 31 matching lines @@
   scoped_refptr<input_method::InputMethodManager::State> saved_ime_state_;
 
   base::WeakPtrFactory<ScreenLocker> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ScreenLocker);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_LOCK_SCREEN_LOCKER_H_