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

chromeos/login/auth/extended_authenticator_impl.h

Index: chromeos/login/auth/extended_authenticator_impl.h
diff --git a/chromeos/login/auth/extended_authenticator_impl.h b/chromeos/login/auth/extended_authenticator_impl.h
index 519c1febc9780bcb4b610fb40a98df533058cac0..4b74299e86951dba42dd098314dd11eb0697e142 100644
--- a/chromeos/login/auth/extended_authenticator_impl.h
+++ b/chromeos/login/auth/extended_authenticator_impl.h
@@ -15,6 +15,10 @@
 #include "chromeos/login/auth/extended_authenticator.h"
 #include "third_party/cros_system_api/dbus/service_constants.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class AuthStatusConsumer;
@@ -32,7 +36,7 @@ class CHROMEOS_EXPORT ExtendedAuthenticatorImpl : public ExtendedAuthenticator {
                            const ResultCallback& success_callback) override;
   void AuthenticateToCheck(const UserContext& context,
                            const base::Closure& success_callback) override;
-  void CreateMount(const std::string& user_id,
+  void CreateMount(const user_manager::UserID& user_id,
                    const std::vector<cryptohome::KeyDefinition>& keys,
                    const ResultCallback& success_callback) override;
   void AddKey(const UserContext& context,