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

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

Index: chrome/browser/ui/webui/chromeos/login/core_oobe_handler.cc
diff --git a/chrome/browser/ui/webui/chromeos/login/core_oobe_handler.cc b/chrome/browser/ui/webui/chromeos/login/core_oobe_handler.cc
index 90b5a8d8956ea61f4e070e7b4387f8d179b2875c..9cedd4cc46c683b68963740152bdfdf0a3ce1608 100644
--- a/chrome/browser/ui/webui/chromeos/login/core_oobe_handler.cc
+++ b/chrome/browser/ui/webui/chromeos/login/core_oobe_handler.cc
@@ -28,6 +28,7 @@
 #include "chrome/grit/generated_resources.h"
 #include "chromeos/chromeos_constants.h"
 #include "components/login/localized_values_builder.h"
+#include "components/user_manager/user_id.h"
 #include "grit/components_strings.h"
 #include "ui/chromeos/accessibility_types.h"
 #include "ui/gfx/display.h"
@@ -190,8 +191,8 @@ void CoreOobeHandler::ShowEnableDebuggingScreen() {
   }
 }
 
-void CoreOobeHandler::ShowSignInUI(const std::string& email) {
-  CallJS("showSigninUI", email);
+void CoreOobeHandler::ShowSignInUI(const user_manager::UserID& user_id) {
+  CallJS("showSigninUI", user_id.GetUserEmail());
 }
 
 void CoreOobeHandler::ResetSignInUI(bool force_online) {
@@ -207,8 +208,8 @@ void CoreOobeHandler::RefocusCurrentPod() {
 }
 
 void CoreOobeHandler::ShowPasswordChangedScreen(bool show_password_error,
-                                                const std::string& email) {
-  CallJS("showPasswordChangedScreen", show_password_error, email);
+                                                const user_manager::UserID& user_id) {
+  CallJS("showPasswordChangedScreen", show_password_error, user_id.GetUserEmail());
 }
 
 void CoreOobeHandler::SetUsageStats(bool checked) {