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

chromeos/login/auth/cryptohome_authenticator.h

Index: chromeos/login/auth/cryptohome_authenticator.h
diff --git a/chromeos/login/auth/cryptohome_authenticator.h b/chromeos/login/auth/cryptohome_authenticator.h
index d767bd950aef0f8e33e858a942b4099043efcdcb..3047e46982dd857d00c8c0b9e126bc746d90eee7 100644
--- a/chromeos/login/auth/cryptohome_authenticator.h
+++ b/chromeos/login/auth/cryptohome_authenticator.h
@@ -134,7 +134,7 @@ class CHROMEOS_EXPORT CryptohomeAuthenticator
   // Otherwise, mounts a public cryptohome, which will be ephemeral if the
   // |DeviceEphemeralUsersEnabled| policy is enabled and non-ephemeral
   // otherwise.
-  void LoginAsKioskAccount(const std::string& app_user_id,
+  void LoginAsKioskAccount(const user_manager::UserID& app_user_id,
                            bool use_guest_mount) override;
 
   // These methods must be called on the UI thread, as they make DBus calls