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

chrome/browser/chromeos/login/ui/models/user_board_model.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_UI_MODELS_USER_BOARD_MODEL_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_UI_MODELS_USER_BOARD_MODEL_H_
 
 #include "chrome/browser/chromeos/login/screens/base_screen.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class UserBoardModel : public BaseScreen {
  public:
   UserBoardModel();
   ~UserBoardModel() override;
 
   // Build list of users and send it to the webui.
   virtual void SendUserList() = 0;
 
   // Methods for easy unlock support.
-  virtual void HardLockPod(const std::string& user_id) = 0;
-  virtual void AttemptEasyUnlock(const std::string& user_id) = 0;
-  virtual void RecordClickOnLockIcon(const std::string& user_id) = 0;
+  virtual void HardLockPod(const user_manager::UserID& user_id) = 0;
+  virtual void AttemptEasyUnlock(const user_manager::UserID& user_id) = 0;
+  virtual void RecordClickOnLockIcon(const user_manager::UserID& user_id) = 0;
 
   // BaseScreen implementation:
   std::string GetName() const override;
 
   // Temorary unused methods:
   void PrepareToShow() override{};
   void Show() override{};
   void Hide() override{};
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_UI_MODELS_USER_BOARD_MODEL_H_