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

chrome/browser/supervised_user/legacy/supervised_user_registration_utility.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 CHROME_BROWSER_SUPERVISED_USER_LEGACY_SUPERVISED_USER_REGISTRATION_UTILITY_H_
 #define CHROME_BROWSER_SUPERVISED_USER_LEGACY_SUPERVISED_USER_REGISTRATION_UTILITY_H_
 
 #include <map>
 #include <string>
 
@@ skipping 13 matching lines @@
 class PrefService;
 class Profile;
 class SupervisedUserRefreshTokenFetcher;
 class SupervisedUserRegistrationUtilityTest;
 class SupervisedUserSharedSettingsService;
 
 namespace browser_sync {
 class DeviceInfo;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 // Structure to store registration information.
 struct SupervisedUserRegistrationInfo {
   SupervisedUserRegistrationInfo(const base::string16& name, int avatar_index);
   ~SupervisedUserRegistrationInfo();
   int avatar_index;
   base::string16 name;
   std::string master_key;
   std::string password_signature_key;
   std::string password_encryption_key;
   base::DictionaryValue password_data;
@@ skipping 11 matching lines @@
   // error for the custodian.
   typedef base::Callback<void(const GoogleServiceAuthError& /* error */,
                               const std::string& /* token */)>
       RegistrationCallback;
 
   virtual ~SupervisedUserRegistrationUtility() {}
 
   // Creates SupervisedUserRegistrationUtility for a given |profile|.
   static scoped_ptr<SupervisedUserRegistrationUtility> Create(Profile* profile);
 
-  static std::string GenerateNewSupervisedUserId();
+  static user_manager::UserID GenerateNewSupervisedUserId();
 
   // Registers a new supervised user with the server. |supervised_user_id| is a
   // new unique ID for the new supervised user. If its value is the same as that
   // of one of the existing supervised users, then the same user will be created
   // on this machine (and if he has no avatar in sync, his avatar will be
   // updated). |info| contains necessary information like the display name of
   // the user and his avatar. |callback| is called with the result of the
   // registration. We use the info here and not the profile, because on Chrome
   // OS the profile of the supervised user does not yet exist.
-  virtual void Register(const std::string& supervised_user_id,
+  virtual void Register(const user_manager::UserID& supervised_user_id,
                         const SupervisedUserRegistrationInfo& info,
                         const RegistrationCallback& callback) = 0;
 
  protected:
   SupervisedUserRegistrationUtility() {}
 
  private:
   friend class ScopedTestingSupervisedUserRegistrationUtility;
   friend class SupervisedUserRegistrationUtilityTest;
 
@@ skipping 15 matching lines @@
 class ScopedTestingSupervisedUserRegistrationUtility {
  public:
   // Delegates ownership of the |instance| to SupervisedUserRegistrationUtility.
   ScopedTestingSupervisedUserRegistrationUtility(
       SupervisedUserRegistrationUtility* instance);
 
   ~ScopedTestingSupervisedUserRegistrationUtility();
 };
 
 #endif  // CHROME_BROWSER_SUPERVISED_USER_LEGACY_SUPERVISED_USER_REGISTRATION_UTILITY_H_