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

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

Index: chrome/browser/chromeos/login/users/multi_profile_user_controller_delegate.h
diff --git a/chrome/browser/chromeos/login/users/multi_profile_user_controller_delegate.h b/chrome/browser/chromeos/login/users/multi_profile_user_controller_delegate.h
index 5509e8445a88b85b351348770c8b31669c087383..72f7a4b169f5aedf4663a9d9e055546640351b66 100644
--- a/chrome/browser/chromeos/login/users/multi_profile_user_controller_delegate.h
+++ b/chrome/browser/chromeos/login/users/multi_profile_user_controller_delegate.h
@@ -5,12 +5,16 @@
 #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() {}