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

chrome/browser/ui/webui/chromeos/login/user_board_screen_handler.h

 // Copyright 2013 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_UI_WEBUI_CHROMEOS_LOGIN_USER_BOARD_SCREEN_HANDLER_H_
 #define CHROME_BROWSER_UI_WEBUI_CHROMEOS_LOGIN_USER_BOARD_SCREEN_HANDLER_H_
 
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/values.h"
 #include "chrome/browser/chromeos/login/ui/views/user_board_view.h"
 #include "chrome/browser/ui/webui/chromeos/login/base_screen_handler.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class SigninScreenHandler;
 
 // A class that handles WebUI hooks in Gaia screen.
 class UserBoardScreenHandler : public BaseScreenHandler, public UserBoardView {
  public:
   UserBoardScreenHandler();
   ~UserBoardScreenHandler() override;
 
  private:
   // BaseScreenHandler implementation:
   void DeclareLocalizedValues(
       ::login::LocalizedValuesBuilder* builder) override;
 
   // WebUIMessageHandler implementation:
   void RegisterMessages() override;
   void Initialize() override;
 
   // Handlers
   void HandleGetUsers();
-  void HandleHardlockPod(const std::string& user_id);
-  void HandleAttemptUnlock(const std::string& user_id);
-  void HandleRecordClickOnLockIcon(const std::string& user_id);
+  void HandleHardlockPod(const std::string& user_email);
+  void HandleAttemptUnlock(const std::string& user_email);
+  void HandleRecordClickOnLockIcon(const std::string& user_email);
 
   // UserBoardView implementation:
-  void SetPublicSessionDisplayName(const std::string& user_id,
+  void SetPublicSessionDisplayName(const user_manager::UserID& user_id,
                                    const std::string& display_name) override;
-  void SetPublicSessionLocales(const std::string& user_id,
+  void SetPublicSessionLocales(const user_manager::UserID& user_id,
                                scoped_ptr<base::ListValue> locales,
                                const std::string& default_locale,
                                bool multiple_recommended_locales) override;
   void ShowBannerMessage(const base::string16& message) override;
-  void ShowUserPodCustomIcon(const std::string& user_id,
+  void ShowUserPodCustomIcon(const user_manager::UserID& user_id,
                              const base::DictionaryValue& icon) override;
-  void HideUserPodCustomIcon(const std::string& user_id) override;
+  void HideUserPodCustomIcon(const user_manager::UserID& user_id) override;
   void SetAuthType(
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       proximity_auth::ScreenlockBridge::LockHandler::AuthType auth_type,
       const base::string16& initial_value) override;
 
   void Bind(UserBoardModel& model) override;
   void Unbind() override;
 
   UserBoardModel* model_;
 
   DISALLOW_COPY_AND_ASSIGN(UserBoardScreenHandler);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_UI_WEBUI_CHROMEOS_LOGIN_USER_BOARD_SCREEN_HANDLER_H_