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

chrome/browser/chromeos/login/lock/screen_locker.cc

Index: chrome/browser/chromeos/login/lock/screen_locker.cc
diff --git a/chrome/browser/chromeos/login/lock/screen_locker.cc b/chrome/browser/chromeos/login/lock/screen_locker.cc
index 2bf45b9e64618e60ae2ddc3c230ac659448f5775..2621a5d49d77938626b9449cfa822db15f7f6812 100644
--- a/chrome/browser/chromeos/login/lock/screen_locker.cc
+++ b/chrome/browser/chromeos/login/lock/screen_locker.cc
@@ -287,12 +287,12 @@ void ScreenLocker::Authenticate(const UserContext& user_context) {
 }
 
 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;
     }
@@ -501,11 +501,11 @@ 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;