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

chromeos/login/auth/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_EXTENDED_AUTHENTICATOR_H_
 #define CHROMEOS_LOGIN_AUTH_EXTENDED_AUTHENTICATOR_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "chromeos/chromeos_export.h"
 #include "chromeos/cryptohome/cryptohome_parameters.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class AuthStatusConsumer;
 class UserContext;
 
 // An interface to interact with cryptohomed: mount home dirs, create new home
 // dirs, update passwords.
 //
 // Typical flow:
 // AuthenticateToMount() calls cryptohomed to perform offline login,
@@ skipping 38 matching lines @@
   // This call will attempt to authenticate the user with the key (and key
   // label) in |context|. No further actions are taken after authentication.
   virtual void AuthenticateToCheck(const UserContext& context,
                                    const base::Closure& success_callback) = 0;
 
   // This call will create and mount the home dir for |user_id| with the given
   // |keys| if the home dir is missing. If the home dir exists already, a mount
   // 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;
 
   // Attempts to add a new |key| for the user identified/authorized by
   // |context|. If a key with the same label already exists, the behavior
   // depends on the |replace_existing| flag. If the flag is set, the old key is
   // replaced. If the flag is not set, an error occurs. It is not allowed to
   // replace the key used for authorization.
   virtual void AddKey(const UserContext& context,
                       const cryptohome::KeyDefinition& key,
@@ skipping 29 matching lines @@
 
  private:
   friend class base::RefCountedThreadSafe<ExtendedAuthenticator>;
 
   DISALLOW_COPY_AND_ASSIGN(ExtendedAuthenticator);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_LOGIN_AUTH_EXTENDED_AUTHENTICATOR_H_