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

chrome/browser/chromeos/login/app_launch_signin_screen.h

 // Copyright 2013 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_APP_LAUNCH_SIGNIN_SCREEN_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_APP_LAUNCH_SIGNIN_SCREEN_H_
 
 #include <string>
 
 #include "base/memory/ref_counted.h"
 #include "chrome/browser/ui/webui/chromeos/login/signin_screen_handler.h"
 #include "chromeos/login/auth/auth_status_consumer.h"
 #include "chromeos/login/auth/authenticator.h"
 #include "components/proximity_auth/screenlock_bridge.h"
 #include "components/user_manager/user.h"
 #include "components/user_manager/user_manager.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class OobeUI;
 
 // The app launch signin screen shows the user pod of the device owner
 // and requires the user to login in order to access the network dialog.
 // This screen is quite similar to the standard lock screen, but we do not
 // create a new view to superimpose over the desktop.
 //
 // TODO(tengs): This class doesn't quite follow the idiom of the other
@@ skipping 22 matching lines @@
   user_manager::UserManager* GetUserManager();
 
   // SigninScreenHandlerDelegate implementation:
   void CancelPasswordChangedFlow() override;
   void CancelUserAdding() override;
   void CreateAccount() override;
   void CompleteLogin(const UserContext& user_context) override;
   void Login(const UserContext& user_context,
              const SigninSpecifics& specifics) override;
   void MigrateUserData(const std::string& old_password) override;
-  void LoadWallpaper(const std::string& username) override;
+  void LoadWallpaper(const user_manager::UserID& user_id) override;
   void LoadSigninWallpaper() override;
   void OnSigninScreenReady() override;
-  void RemoveUser(const std::string& username) override;
+  void RemoveUser(const user_manager::UserID& user_id) override;
   void ResyncUserData() 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);
   const user_manager::UserList& GetUsers() const override;
   bool IsShowGuest() const override;
   bool IsShowUsers() const override;
   bool IsSigninInProgress() const override;
   bool IsUserSigninCompleted() const override;
   void SetDisplayEmail(const std::string& email) override;
   void Signout() override;
   void HandleGetUsers() override;
-  void CheckUserStatus(const std::string& user_id) override;
+  void CheckUserStatus(const user_manager::UserID& user_id) override;
 
   // AuthStatusConsumer implementation:
   void OnAuthFailure(const AuthFailure& error) override;
   void OnAuthSuccess(const UserContext& user_context) override;
 
   OobeUI* oobe_ui_;
   Delegate* delegate_;
   LoginDisplayWebUIHandler* webui_handler_;
   scoped_refptr<Authenticator> authenticator_;
 
   // This list should have at most one user, and that user should be the owner.
   user_manager::UserList owner_user_list_;
 
   static user_manager::UserManager* test_user_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(AppLaunchSigninScreen);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_APP_LAUNCH_SIGNIN_SCREEN_H_