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

chrome/browser/chromeos/login/ui/webui_login_display.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_UI_WEBUI_LOGIN_DISPLAY_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_UI_WEBUI_LOGIN_DISPLAY_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 17 matching lines @@
   explicit WebUILoginDisplay(LoginDisplay::Delegate* delegate);
   ~WebUILoginDisplay() override;
 
   // LoginDisplay implementation:
   void ClearAndEnablePassword() override;
   void Init(const user_manager::UserList& users,
             bool show_guest,
             bool show_users,
             bool show_new_user) override;
   void OnPreferencesChanged() override;
-  void RemoveUser(const std::string& user_id) override;
+  void RemoveUser(const user_manager::UserID& user_id) override;
   void SetUIEnabled(bool is_enabled) override;
   void ShowError(int error_msg_id,
                  int login_attempts,
                  HelpAppLauncher::HelpTopic help_topic_id) override;
   void ShowErrorScreen(LoginDisplay::SigninError error_id) override;
-  void ShowGaiaPasswordChanged(const std::string& username) override;
+  void ShowGaiaPasswordChanged(const user_manager::UserID& user_id) override;
   void ShowPasswordChangedDialog(bool show_password_error,
-                                 const std::string& email) override;
-  void ShowSigninUI(const std::string& email) override;
+                                 const user_manager::UserID& user_id) override;
+  void ShowSigninUI(const user_manager::UserID& user_id) override;
   void ShowWhitelistCheckFailedError() override;
 
   // NativeWindowDelegate implementation:
   gfx::NativeWindow GetNativeWindow() const override;
 
   // SigninScreenHandlerDelegate implementation:
   void CancelPasswordChangedFlow() override;
   void ResyncUserData() override;
   void MigrateUserData(const std::string& old_password) override;
 
   void Login(const UserContext& user_context,
              const SigninSpecifics& specifics) override;
   bool IsSigninInProgress() const override;
   void Signout() override;
   void CreateAccount() override;
   void CompleteLogin(const UserContext& user_context) override;
 
   void OnSigninScreenReady() override;
   void CancelUserAdding() override;
-  void LoadWallpaper(const std::string& username) override;
+  void LoadWallpaper(const user_manager::UserID& user_id) override;
   void LoadSigninWallpaper() override;
   void ShowEnterpriseEnrollmentScreen() override;
   void ShowEnableDebuggingScreen() override;
   void ShowKioskEnableScreen() override;
   void ShowKioskAutolaunchScreen() override;
   void ShowWrongHWIDScreen() override;
   void SetWebUIHandler(LoginDisplayWebUIHandler* webui_handler) override;
-  virtual void ShowSigninScreenForCreds(const std::string& username,
+  virtual void ShowSigninScreenForCreds(const user_manager::UserID& user_id,
                                         const std::string& password);
   bool IsShowGuest() const override;
   bool IsShowUsers() const override;
   bool IsUserSigninCompleted() const override;
   void SetDisplayEmail(const std::string& email) override;
 
   void HandleGetUsers() override;
-  void CheckUserStatus(const std::string& user_id) override;
+  void CheckUserStatus(const user_manager::UserID& user_id) override;
   const user_manager::UserList& GetUsers() const override;
 
   // ui::UserActivityDetector implementation:
   void OnUserActivity(const ui::Event* event) override;
 
  private:
 
   // Whether to show guest login.
   bool show_guest_;
 
   // Weather to show the user pods or a GAIA sign in.
   // Public sessions are always shown.
   bool show_users_;
 
   // Whether to show add new user.
   bool show_new_user_;
 
   // Reference to the WebUI handling layer for the login screen
   LoginDisplayWebUIHandler* webui_handler_;
 
   DISALLOW_COPY_AND_ASSIGN(WebUILoginDisplay);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_UI_WEBUI_LOGIN_DISPLAY_H_