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

chrome/browser/chromeos/login/screens/chrome_user_selection_screen.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_SCREENS_CHROME_USER_SELECTION_SCREEN_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_CHROME_USER_SELECTION_SCREEN_H_
 
 #include <map>
 #include <string>
 #include <vector>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/chromeos/login/screens/user_selection_screen.h"
 #include "chrome/browser/chromeos/policy/device_local_account_policy_service.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class ChromeUserSelectionScreen
     : public UserSelectionScreen,
       public policy::DeviceLocalAccountPolicyService::Observer {
  public:
   explicit ChromeUserSelectionScreen(const std::string& display_type);
   ~ChromeUserSelectionScreen() override;
 
   // UserSelectionScreen:
   void Init(const user_manager::UserList& users, bool show_guest) override;
   void SendUserList() override;
 
   // policy::DeviceLocalAccountPolicyService::Observer:
-  void OnPolicyUpdated(const std::string& user_id) override;
+  void OnPolicyUpdated(const user_manager::UserID& user_id) override;
   void OnDeviceLocalAccountsChanged() override;
 
  private:
   // Check whether the display name set by policy for a public session has
   // changed and if so, notify the UI.
   void CheckForPublicSessionDisplayNameChange(
       policy::DeviceLocalAccountPolicyBroker* broker);
 
   // Check whether the list of recommended locales set by policy for a public
   // session has changed and if so, notify the UI.
   void CheckForPublicSessionLocalePolicyChange(
       policy::DeviceLocalAccountPolicyBroker* broker);
 
   // Notify the UI that the display name for a public session has changed,
   // taking the display name from the |User| owned by |UserManager|.
-  void SetPublicSessionDisplayName(const std::string& user_id);
+  void SetPublicSessionDisplayName(const user_manager::UserID& user_id);
 
   // Send an updated list of locales for a public session to the UI, consisting
   // of the |recommended_locales| followed by all other available locales.
   void SetPublicSessionLocales(
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       const std::vector<std::string>& recommended_locales);
 
   bool handler_initialized_;
 
   policy::DeviceLocalAccountPolicyService* device_local_account_policy_service_;
 
   // Map from public session user IDs to their display names set by policy.
-  typedef std::map<std::string, std::string> DisplayNamesMap;
+  typedef std::map<user_manager::UserID, std::string> DisplayNamesMap;
   DisplayNamesMap public_session_display_names_;
 
   base::WeakPtrFactory<ChromeUserSelectionScreen> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeUserSelectionScreen);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_CHROME_USER_SELECTION_SCREEN_H_