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

chrome/browser/chromeos/login/ui/views/user_board_view.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_VIEWS_USER_BOARD_VIEW_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_UI_VIEWS_USER_BOARD_VIEW_H_
 
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/string16.h"
 #include "base/values.h"
 #include "components/proximity_auth/screenlock_bridge.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class UserBoardModel;
 
 // Interface between user board screen and its representation, either WebUI
 // or Views one.
 class UserBoardView {
  public:
   virtual ~UserBoardView() {}
 
   virtual void Bind(UserBoardModel& model) = 0;
   virtual void Unbind() = 0;
 
-  virtual void SetPublicSessionDisplayName(const std::string& user_id,
+  virtual void SetPublicSessionDisplayName(const user_manager::UserID& user_id,
                                            const std::string& display_name) = 0;
-  virtual void SetPublicSessionLocales(const std::string& user_id,
+  virtual void SetPublicSessionLocales(const user_manager::UserID& user_id,
                                        scoped_ptr<base::ListValue> locales,
                                        const std::string& default_locale,
                                        bool multiple_recommended_locales) = 0;
   virtual void ShowBannerMessage(const base::string16& message) = 0;
-  virtual void ShowUserPodCustomIcon(const std::string& user_id,
+  virtual void ShowUserPodCustomIcon(const user_manager::UserID& user_id,
                                      const base::DictionaryValue& icon) = 0;
-  virtual void HideUserPodCustomIcon(const std::string& user_id) = 0;
+  virtual void HideUserPodCustomIcon(const user_manager::UserID& user_id) = 0;
   virtual 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) = 0;
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_UI_VIEWS_USER_BOARD_VIEW_H_