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

chromeos/login/auth/stub_authenticator.h

Index: chromeos/login/auth/stub_authenticator.h
diff --git a/chromeos/login/auth/stub_authenticator.h b/chromeos/login/auth/stub_authenticator.h
index 0fa11c4de60c31bb8bc09753a6f61b86ad9ecd3c..aaab1a479e1176bcefa871a17ab7e73918983c3d 100644
--- a/chromeos/login/auth/stub_authenticator.h
+++ b/chromeos/login/auth/stub_authenticator.h
@@ -16,6 +16,10 @@ namespace content {
 class BrowserContext;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class AuthStatusConsumer;
@@ -34,7 +38,7 @@ class CHROMEOS_EXPORT StubAuthenticator : public Authenticator {
   void LoginAsSupervisedUser(const UserContext& user_context) override;
   void LoginOffTheRecord() override;
   void LoginAsPublicSession(const UserContext& user_context) override;
-  void LoginAsKioskAccount(const std::string& app_user_id,
+  void LoginAsKioskAccount(const user_manager::UserID& app_user_id,
                            bool use_guest_mount) override;
   void OnAuthSuccess() override;
   void OnAuthFailure(const AuthFailure& failure) override;