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

chrome/browser/chromeos/login/screens/error_screen.h

 // Copyright (c) 2012 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_SCREENS_ERROR_SCREEN_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_ERROR_SCREEN_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 39 matching lines @@
   void SetParentScreen(OobeUI::Screen parent_screen) override;
   void SetHideCallback(const base::Closure& on_hide) override;
   void ShowCaptivePortal() override;
   void ShowConnectingIndicator(bool show) override;
 
   // LoginPerformer::Delegate implementation:
   void OnAuthFailure(const AuthFailure& error) override;
   void OnAuthSuccess(const UserContext& user_context) override;
   void OnOffTheRecordAuthSuccess() override;
   void OnPasswordChangeDetected() override;
-  void WhiteListCheckFailed(const std::string& email) override;
+  void WhiteListCheckFailed(const user_manager::UserID& user_id) override;
   void PolicyLoadFailed() override;
-  void OnOnlineChecked(const std::string& username, bool success) override;
+  void OnOnlineChecked(const user_manager::UserID& user_id, bool success) override;
 
  private:
   // Default hide_closure for Hide().
   void DefaultHideCallback();
 
   // Handle user action to configure certificates.
   void OnConfigureCerts();
 
   // Handle user action to diagnose network configuration.
   void OnDiagnoseButtonClicked();
@@ skipping 32 matching lines @@
   scoped_ptr<base::Closure> on_hide_callback_;
 
   base::WeakPtrFactory<ErrorScreen> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ErrorScreen);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_ERROR_SCREEN_H_