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

chromeos/login/auth/fake_extended_authenticator.h

Index: chromeos/login/auth/fake_extended_authenticator.h
diff --git a/chromeos/login/auth/fake_extended_authenticator.h b/chromeos/login/auth/fake_extended_authenticator.h
index 96dc4f497c36b4bb94d773aa5b967ea80bb57880..d4a0ff4375156aa00979b0f9c0ec24637a3f7a83 100644
--- a/chromeos/login/auth/fake_extended_authenticator.h
+++ b/chromeos/login/auth/fake_extended_authenticator.h
@@ -10,6 +10,10 @@
 #include "chromeos/login/auth/extended_authenticator.h"
 #include "chromeos/login/auth/user_context.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class AuthFailure;
@@ -27,7 +31,7 @@ class CHROMEOS_EXPORT FakeExtendedAuthenticator : 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,