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

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

Index: chrome/browser/chromeos/login/lock/screen_locker.h
diff --git a/chrome/browser/chromeos/login/lock/screen_locker.h b/chrome/browser/chromeos/login/lock/screen_locker.h
index 1708dae40287c10372737efa43a78b2048e64124..eb4d02aa41169200fa87ffa0f5b611ba32696f51 100644
--- a/chrome/browser/chromeos/login/lock/screen_locker.h
+++ b/chrome/browser/chromeos/login/lock/screen_locker.h
@@ -141,11 +141,11 @@ class ScreenLocker : public AuthStatusConsumer {
   // Called when screen locker is safe to delete.
   static void ScheduleDeletion();
 
-  // Returns true if |username| is found among logged in users.
-  bool IsUserLoggedIn(const std::string& username);
+  // Returns true if |user_id| is found among logged in users.
+  bool IsUserLoggedIn(const user_manager::UserID& user_id);
 
   // Looks up user in unlock user list.
-  const user_manager::User* FindUnlockUser(const std::string& user_id);
+  const user_manager::User* FindUnlockUser(const user_manager::UserID& user_id);
 
   // ScreenLockerDelegate instance in use.
   scoped_ptr<ScreenLockerDelegate> delegate_;