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

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

Index: chrome/browser/chromeos/login/screens/core_oobe_actor.h
diff --git a/chrome/browser/chromeos/login/screens/core_oobe_actor.h b/chrome/browser/chromeos/login/screens/core_oobe_actor.h
index 6a491d2c088f068fbdfa9c50a63e4247f86c711f..501e96418877557e0cf3a61f632dbe70fc5f4e23 100644
--- a/chrome/browser/chromeos/login/screens/core_oobe_actor.h
+++ b/chrome/browser/chromeos/login/screens/core_oobe_actor.h
@@ -17,6 +17,10 @@ namespace gfx {
 class Rect;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class CoreOobeActor {
@@ -28,12 +32,12 @@ class CoreOobeActor {
                                const std::string& help_link_text,
                                HelpAppLauncher::HelpTopic help_topic_id) = 0;
   virtual void ShowTpmError() = 0;
-  virtual void ShowSignInUI(const std::string& email) = 0;
+  virtual void ShowSignInUI(const user_manager::UserID& user_id) = 0;
   virtual void ResetSignInUI(bool force_online) = 0;
   virtual void ClearUserPodPassword() = 0;
   virtual void RefocusCurrentPod() = 0;
   virtual void ShowPasswordChangedScreen(bool show_password_error,
-                                         const std::string& email) = 0;
+                                         const user_manager::UserID& user_id) = 0;
   virtual void SetUsageStats(bool checked) = 0;
   virtual void SetOemEulaUrl(const std::string& oem_eula_url) = 0;
   virtual void SetTpmPassword(const std::string& tmp_password) = 0;