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

chrome/browser/chromeos/login/user_flow.cc

 // Copyright (c) 2012 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 "base/bind.h"
 #include "base/message_loop/message_loop.h"
 #include "chrome/browser/chromeos/login/user_flow.h"
 #include "chrome/browser/chromeos/login/users/chrome_user_manager.h"
 
 namespace chromeos {
 
 namespace {
 
-void UnregisterFlow(const std::string& user_id) {
+void UnregisterFlow(const user_manager::UserID& user_id) {
   ChromeUserManager::Get()->ResetUserFlow(user_id);
 }
 
 } // namespace
 
 
 UserFlow::UserFlow() : host_(NULL) {}
 
 UserFlow::~UserFlow() {}
 
@@ skipping 38 matching lines @@
   return false;
 }
 
 void DefaultUserFlow::HandleOAuthTokenStatusChange(
     user_manager::User::OAuthTokenStatus status) {
 }
 
 void DefaultUserFlow::LaunchExtraSteps(Profile* profile) {
 }
 
-ExtendedUserFlow::ExtendedUserFlow(const std::string& user_id)
+ExtendedUserFlow::ExtendedUserFlow(const user_manager::UserID& user_id)
     : user_id_(user_id) {
 }
 
 ExtendedUserFlow::~ExtendedUserFlow() {
 }
 
 void ExtendedUserFlow::AppendAdditionalCommandLineSwitches() {
 }
 
 bool ExtendedUserFlow::ShouldShowSettings() {
   return true;
 }
 
 void ExtendedUserFlow::HandleOAuthTokenStatusChange(
     user_manager::User::OAuthTokenStatus status) {
 }
 
 void ExtendedUserFlow::UnregisterFlowSoon() {
-  std::string id_copy(user_id());
+  user_manager::UserID id_copy(user_id());
   base::MessageLoop::current()->PostTask(FROM_HERE,
       base::Bind(&UnregisterFlow,
                  id_copy));
 }
 
 }  // namespace chromeos