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

chrome/browser/chromeos/login/app_launch_signin_screen.h

Index: chrome/browser/chromeos/login/app_launch_signin_screen.h
diff --git a/chrome/browser/chromeos/login/app_launch_signin_screen.h b/chrome/browser/chromeos/login/app_launch_signin_screen.h
index 7a9bb77ab32cd3f3549531d899af97780ffe8779..9a8d90f52b8ca5e020d47b61c0ce88041b85ddd1 100644
--- a/chrome/browser/chromeos/login/app_launch_signin_screen.h
+++ b/chrome/browser/chromeos/login/app_launch_signin_screen.h
@@ -15,6 +15,10 @@
 #include "components/user_manager/user.h"
 #include "components/user_manager/user_manager.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class OobeUI;
@@ -57,10 +61,10 @@ class AppLaunchSigninScreen : public SigninScreenHandlerDelegate,
   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;
@@ -68,7 +72,7 @@ class AppLaunchSigninScreen : public SigninScreenHandlerDelegate,
   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;
@@ -78,7 +82,7 @@ class AppLaunchSigninScreen : public SigninScreenHandlerDelegate,
   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;