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

chromeos/login/auth/fake_extended_authenticator.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROMEOS_LOGIN_AUTH_FAKE_EXTENDED_AUTHENTICATOR_H_
 #define CHROMEOS_LOGIN_AUTH_FAKE_EXTENDED_AUTHENTICATOR_H_
 
 #include "base/macros.h"
 #include "chromeos/chromeos_export.h"
 #include "chromeos/login/auth/extended_authenticator.h"
 #include "chromeos/login/auth/user_context.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class AuthFailure;
 
 class CHROMEOS_EXPORT FakeExtendedAuthenticator : public ExtendedAuthenticator {
  public:
   FakeExtendedAuthenticator(NewAuthStatusConsumer* consumer,
                             const UserContext& expected_user_context);
   FakeExtendedAuthenticator(AuthStatusConsumer* consumer,
                             const UserContext& expected_user_context);
 
   // ExtendedAuthenticator:
   void SetConsumer(AuthStatusConsumer* consumer) override;
   void AuthenticateToMount(const UserContext& context,
                            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,
               const cryptohome::KeyDefinition& key,
               bool replace_existing,
               const base::Closure& success_callback) override;
   void UpdateKeyAuthorized(const UserContext& context,
                            const cryptohome::KeyDefinition& key,
                            const std::string& signature,
                            const base::Closure& success_callback) override;
@@ skipping 13 matching lines @@
   AuthStatusConsumer* old_consumer_;
 
   UserContext expected_user_context_;
 
   DISALLOW_COPY_AND_ASSIGN(FakeExtendedAuthenticator);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_LOGIN_AUTH_FAKE_EXTENDED_AUTHENTICATOR_H_