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

chrome/browser/chromeos/login/signin_screen_controller.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 "chrome/browser/chromeos/login/signin_screen_controller.h"
 
 #include "chrome/browser/chrome_notification_types.h"
 #include "chrome/browser/chromeos/login/lock/screen_locker.h"
 #include "chrome/browser/chromeos/login/screens/chrome_user_selection_screen.h"
 #include "chrome/browser/chromeos/login/ui/views/user_board_view.h"
@@ skipping 40 matching lines @@
 }
 
 void SignInScreenController::OnSigninScreenReady() {
   gaia_screen_->MaybePreloadAuthExtension();
   user_selection_screen_->InitEasyUnlock();
   if (ScreenLocker::default_screen_locker()) {
     ScreenLocker::default_screen_locker()->delegate()->OnLockWebUIReady();
   }
 }
 
-void SignInScreenController::RemoveUser(const std::string& user_id) {
+void SignInScreenController::RemoveUser(const user_manager::UserID& user_id) {
   user_manager::UserManager::Get()->RemoveUser(user_id, this);
 }
 
-void SignInScreenController::OnBeforeUserRemoved(const std::string& username) {
-  user_selection_screen_->OnBeforeUserRemoved(username);
+void SignInScreenController::OnBeforeUserRemoved(const user_manager::UserID& user_id) {
+  user_selection_screen_->OnBeforeUserRemoved(user_id);
 }
 
-void SignInScreenController::OnUserRemoved(const std::string& username) {
-  user_selection_screen_->OnUserRemoved(username);
+void SignInScreenController::OnUserRemoved(const user_manager::UserID& user_id) {
+  user_selection_screen_->OnUserRemoved(user_id);
 }
 
 void SignInScreenController::SendUserList() {
   user_selection_screen_->HandleGetUsers();
 }
 
 const user_manager::UserList& SignInScreenController::GetUsers() {
   return user_selection_screen_->GetUsers();
 }
 
-void SignInScreenController::CheckUserStatus(const std::string& user_id) {
+void SignInScreenController::CheckUserStatus(const user_manager::UserID& user_id) {
   user_selection_screen_->CheckUserStatus(user_id);
 }
 
 void SignInScreenController::SetWebUIHandler(
     LoginDisplayWebUIHandler* webui_handler) {
   webui_handler_ = webui_handler;
   gaia_screen_->SetLegacyHandler(webui_handler_);
   user_selection_screen_->SetHandler(webui_handler_);
 }
 
 void SignInScreenController::Observe(
     int type,
     const content::NotificationSource& source,
     const content::NotificationDetails& details) {
   if (type == chrome::NOTIFICATION_SESSION_STARTED) {
     // Stop listening to any notification once session has started.
     // Sign in screen objects are marked for deletion with DeleteSoon so
     // make sure no object would be used after session has started.
     // http://crbug.com/125276
     registrar_.RemoveAll();
     return;
   } else if (type == chrome::NOTIFICATION_LOGIN_USER_IMAGE_CHANGED) {
     user_selection_screen_->OnUserImageChanged(
         *content::Details<user_manager::User>(details).ptr());
     return;
   }
 }
 
 }  // namespace chromeos