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

chrome/browser/supervised_user/legacy/supervised_user_registration_utility_stub.cc

 // 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.
 
 #include "chrome/browser/supervised_user/legacy/supervised_user_registration_utility_stub.h"
 
 #include "base/bind.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/utf_string_conversions.h"
+#include "components/user_manager/user_id.h"
 #include "google_apis/gaia/gaia_urls.h"
 #include "google_apis/gaia/google_service_auth_error.h"
 
 SupervisedUserRegistrationUtilityStub::SupervisedUserRegistrationUtilityStub()
-    : register_was_called_(false) {
+    : register_was_called_(false), supervised_user_id_(std::string(), std::string()) {
 }
 
 SupervisedUserRegistrationUtilityStub::
 ~SupervisedUserRegistrationUtilityStub() {
 }
 
 void SupervisedUserRegistrationUtilityStub::Register(
-    const std::string& supervised_user_id,
+    const user_manager::UserID& supervised_user_id,
     const SupervisedUserRegistrationInfo& info,
     const RegistrationCallback& callback) {
   DCHECK(!register_was_called_);
   register_was_called_ = true;
   callback_ = callback;
   supervised_user_id_ = supervised_user_id;
   display_name_ = info.name;
   master_key_ = info.master_key;
 }
 
 void SupervisedUserRegistrationUtilityStub::RunSuccessCallback(
     const std::string& token) {
   if (callback_.is_null())
     return;
   callback_.Run(GoogleServiceAuthError(GoogleServiceAuthError::NONE), token);
   callback_.Reset();
 }
 
 void SupervisedUserRegistrationUtilityStub::RunFailureCallback(
     const GoogleServiceAuthError::State state) {
   if (callback_.is_null())
     return;
   GoogleServiceAuthError error(state);
   callback_.Run(error, std::string());
   callback_.Reset();
 }