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

chrome/browser/chromeos/login/signin_screen_controller.h

 // Copyright (c) 2012 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_SIGNIN_SCREEN_CONTROLLER_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SIGNIN_SCREEN_CONTROLLER_H_
 
 #include "chrome/browser/chromeos/login/screens/gaia_screen.h"
 #include "chrome/browser/chromeos/login/screens/user_selection_screen.h"
 #include "chrome/browser/chromeos/login/ui/login_display_host.h"
@@ skipping 28 matching lines @@
   // Called when signin screen is ready.
   void OnSigninScreenReady();
 
   // Query to send list of users to user selection screen.
   void SendUserList();
 
   // Provide current list of users on user selection screen.
   const user_manager::UserList& GetUsers();
 
   // Runs OAauth token validity check.
-  void CheckUserStatus(const std::string& user_id);
+  void CheckUserStatus(const user_manager::UserID& user_id);
 
   // Query to remove user with specified id.
   // TODO(antrim): move to user selection screen handler.
-  void RemoveUser(const std::string& user_id);
+  void RemoveUser(const user_manager::UserID& user_id);
 
   // user_manager::RemoveUserDelegate implementation:
-  void OnBeforeUserRemoved(const std::string& username) override;
-  void OnUserRemoved(const std::string& username) override;
+  void OnBeforeUserRemoved(const user_manager::UserID& user_id) override;
+  void OnUserRemoved(const user_manager::UserID& user_id) override;
 
   // content::NotificationObserver implementation.
   void Observe(int type,
                const content::NotificationSource& source,
                const content::NotificationDetails& details) override;
 
  private:
   static SignInScreenController* instance_;
 
   OobeDisplay* oobe_display_;
 
   // Reference to the WebUI handling layer for the login screen
   LoginDisplayWebUIHandler* webui_handler_;
 
   scoped_ptr<GaiaScreen> gaia_screen_;
   scoped_ptr<UserSelectionScreen> user_selection_screen_;
 
   // Used for notifications during the login process.
   content::NotificationRegistrar registrar_;
 
   DISALLOW_COPY_AND_ASSIGN(SignInScreenController);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SIGNIN_SCREEN_CONTROLLER_H_