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

chrome/browser/chromeos/login/lock/screen_locker.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/chromeos/login/lock/screen_locker.h"
 
 #include <string>
 #include <vector>
 
 #include "ash/ash_switches.h"
@@ skipping 269 matching lines @@
 
   BrowserThread::PostTask(
       BrowserThread::UI, FROM_HERE,
       base::Bind(&ExtendedAuthenticator::AuthenticateToCheck,
                  extended_authenticator_.get(),
                  user_context,
                  base::Closure()));
 }
 
 const user_manager::User* ScreenLocker::FindUnlockUser(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   const user_manager::User* unlock_user = NULL;
   for (user_manager::UserList::const_iterator it = users_.begin();
        it != users_.end();
        ++it) {
-    if ((*it)->email() == user_id) {
+    if ((*it)->GetUserID() == user_id) {
       unlock_user = *it;
       break;
     }
   }
   return unlock_user;
 }
 
 void ScreenLocker::ClearErrors() {
   delegate_->ClearErrors();
 }
@@ skipping 188 matching lines @@
 
   input_method::InputMethodManager::Get()
       ->GetActiveIMEState()
       ->EnableLockScreenLayouts();
 }
 
 content::WebUI* ScreenLocker::GetAssociatedWebUI() {
   return delegate_->GetAssociatedWebUI();
 }
 
-bool ScreenLocker::IsUserLoggedIn(const std::string& username) {
+bool ScreenLocker::IsUserLoggedIn(const user_manager::UserID& user_id) {
   for (user_manager::UserList::const_iterator it = users_.begin();
        it != users_.end();
        ++it) {
-    if ((*it)->email() == username)
+    if ((*it)->GetUserID() == user_id)
       return true;
   }
   return false;
 }
 
 }  // namespace chromeos