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

chrome/browser/supervised_user/legacy/supervised_user_registration_utility_stub.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_STUB_H_
 #define CHROME_BROWSER_SUPERVISED_USER_LEGACY_SUPERVISED_USER_REGISTRATION_UTILITY_STUB_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/strings/string16.h"
 #include "chrome/browser/supervised_user/legacy/supervised_user_registration_utility.h"
+#include "components/user_manager/user_id.h"
 #include "google_apis/gaia/google_service_auth_error.h"
 
 class SupervisedUserRegistrationUtilityStub
     : public SupervisedUserRegistrationUtility {
  public:
   SupervisedUserRegistrationUtilityStub();
   ~SupervisedUserRegistrationUtilityStub() override;
 
-  void Register(const std::string& supervised_user_id,
+  void Register(const user_manager::UserID& supervised_user_id,
                 const SupervisedUserRegistrationInfo& info,
                 const RegistrationCallback& callback) override;
 
   bool register_was_called() { return register_was_called_; }
 
-  std::string supervised_user_id() { return supervised_user_id_; }
+  user_manager::UserID supervised_user_id() { return supervised_user_id_; }
 
   base::string16 display_name() { return display_name_; }
 
   std::string master_key() { return master_key_; }
 
   void RunSuccessCallback(const std::string& token);
   void RunFailureCallback(GoogleServiceAuthError::State error);
 
  private:
   RegistrationCallback callback_;
   bool register_was_called_;
-  std::string supervised_user_id_;
+  user_manager::UserID supervised_user_id_;
   base::string16 display_name_;
   std::string master_key_;
 
   DISALLOW_COPY_AND_ASSIGN(SupervisedUserRegistrationUtilityStub);
 };
 
 #endif  // CHROME_BROWSER_SUPERVISED_USER_LEGACY_SUPERVISED_USER_REGISTRATION_UTILITY_STUB_H_