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

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

Index: chrome/browser/chromeos/login/screens/chrome_user_selection_screen.h
diff --git a/chrome/browser/chromeos/login/screens/chrome_user_selection_screen.h b/chrome/browser/chromeos/login/screens/chrome_user_selection_screen.h
index e276911a1136baadd97a1ee5c4691b46335728b8..e6bfbf4a93a43b0326e1ace21ec231f8ad3d0796 100644
--- a/chrome/browser/chromeos/login/screens/chrome_user_selection_screen.h
+++ b/chrome/browser/chromeos/login/screens/chrome_user_selection_screen.h
@@ -15,6 +15,10 @@
 #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
@@ -29,7 +33,7 @@ class ChromeUserSelectionScreen
   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:
@@ -45,12 +49,12 @@ class ChromeUserSelectionScreen
 
   // 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_;
@@ -58,7 +62,7 @@ class ChromeUserSelectionScreen
   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_;