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

chromeos/login/auth/extended_authenticator.h

Index: chromeos/login/auth/extended_authenticator.h
diff --git a/chromeos/login/auth/extended_authenticator.h b/chromeos/login/auth/extended_authenticator.h
index d80b20932c5b7855e5f054bc4da8ce4afa1a7b21..7b0fce5fbd21f7344eaeed52c444549f7853dd19 100644
--- a/chromeos/login/auth/extended_authenticator.h
+++ b/chromeos/login/auth/extended_authenticator.h
@@ -15,6 +15,10 @@
 #include "chromeos/chromeos_export.h"
 #include "chromeos/cryptohome/cryptohome_parameters.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class AuthStatusConsumer;
@@ -73,7 +77,7 @@ class CHROMEOS_EXPORT ExtendedAuthenticator
   // attempt will be performed using the first key in |keys| for authentication.
   // Note that all |keys| should have been transformed from plain text already.
   // This method does not alter them.
-  virtual void CreateMount(const std::string& user_id,
+  virtual void CreateMount(const user_manager::UserID& user_id,
                            const std::vector<cryptohome::KeyDefinition>& keys,
                            const ResultCallback& success_callback) = 0;