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

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

Index: chrome/browser/ui/webui/chromeos/login/core_oobe_handler.h
diff --git a/chrome/browser/ui/webui/chromeos/login/core_oobe_handler.h b/chrome/browser/ui/webui/chromeos/login/core_oobe_handler.h
index c0b1073def7e5b320ea3f9203b8016f0a22890a2..0997cd853374f842aaebd27a3c0aa10cdc034ad3 100644
--- a/chrome/browser/ui/webui/chromeos/login/core_oobe_handler.h
+++ b/chrome/browser/ui/webui/chromeos/login/core_oobe_handler.h
@@ -21,6 +21,10 @@ namespace gfx {
 class Rect;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class HelpAppLauncher;
@@ -75,12 +79,12 @@ class CoreOobeHandler : public BaseScreenHandler,
                        const std::string& help_link_text,
                        HelpAppLauncher::HelpTopic help_topic_id) override;
   void ShowTpmError() override;
-  void ShowSignInUI(const std::string& email) override;
+  void ShowSignInUI(const user_manager::UserID& user_id) override;
   void ResetSignInUI(bool force_online) override;
   void ClearUserPodPassword() override;
   void RefocusCurrentPod() override;
   void ShowPasswordChangedScreen(bool show_password_error,
-                                 const std::string& email) override;
+                                 const user_manager::UserID& user_id) override;
   void SetUsageStats(bool checked) override;
   void SetOemEulaUrl(const std::string& oem_eula_url) override;
   void SetTpmPassword(const std::string& tmp_password) override;