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

chrome/browser/chromeos/login/users/multi_profile_user_controller_delegate.h

 // 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.
 
 #ifndef CHROME_BROWSER_CHROMEOS_LOGIN_USERS_MULTI_PROFILE_USER_CONTROLLER_DELEGATE_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_USERS_MULTI_PROFILE_USER_CONTROLLER_DELEGATE_H_
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class MultiProfileUserControllerDelegate {
  public:
   // Invoked when the first user that is not allowed in the session is detected.
-  virtual void OnUserNotAllowed(const std::string& user_email) = 0;
+  virtual void OnUserNotAllowed(const user_manager::UserID& user_id) = 0;
 
  protected:
   virtual ~MultiProfileUserControllerDelegate() {}
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_USERS_MULTI_PROFILE_USER_CONTROLLER_DELEGATE_H_