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

chrome/browser/chromeos/login/ui/webui_login_display.cc

Index: chrome/browser/chromeos/login/ui/webui_login_display.cc
diff --git a/chrome/browser/chromeos/login/ui/webui_login_display.cc b/chrome/browser/chromeos/login/ui/webui_login_display.cc
index 264ef1457627ac7a85191c85ce8faf823e9490c4..f0fee0be0159b40f7e2e0d0d5db004a48fa96ba4 100644
--- a/chrome/browser/chromeos/login/ui/webui_login_display.cc
+++ b/chrome/browser/chromeos/login/ui/webui_login_display.cc
@@ -80,7 +80,7 @@ const user_manager::UserList& WebUILoginDisplay::GetUsers() const {
   return SignInScreenController::Get()->GetUsers();
 }
 
-void WebUILoginDisplay::CheckUserStatus(const std::string& user_id) {
+void WebUILoginDisplay::CheckUserStatus(const user_manager::UserID& user_id) {
   SignInScreenController::Get()->CheckUserStatus(user_id);
 }
 
@@ -180,20 +180,20 @@ void WebUILoginDisplay::ShowErrorScreen(LoginDisplay::SigninError error_id) {
   webui_handler_->ShowErrorScreen(error_id);
 }
 
-void WebUILoginDisplay::ShowGaiaPasswordChanged(const std::string& username) {
+void WebUILoginDisplay::ShowGaiaPasswordChanged(const user_manager::UserID& user_id) {
   if (webui_handler_)
-    webui_handler_->ShowGaiaPasswordChanged(username);
+    webui_handler_->ShowGaiaPasswordChanged(user_id);
 }
 
 void WebUILoginDisplay::ShowPasswordChangedDialog(bool show_password_error,
-                                                  const std::string& email) {
+                                                  const user_manager::UserID& user_id) {
   if (webui_handler_)
-    webui_handler_->ShowPasswordChangedDialog(show_password_error, email);
+    webui_handler_->ShowPasswordChangedDialog(show_password_error, user_id);
 }
 
-void WebUILoginDisplay::ShowSigninUI(const std::string& email) {
+void WebUILoginDisplay::ShowSigninUI(const user_manager::UserID& user_id) {
   if (webui_handler_)
-    webui_handler_->ShowSigninUI(email);
+    webui_handler_->ShowSigninUI(user_id);
 }
 
 void WebUILoginDisplay::ShowWhitelistCheckFailedError() {
@@ -246,13 +246,13 @@ void WebUILoginDisplay::MigrateUserData(const std::string& old_password) {
     delegate_->MigrateUserData(old_password);
 }
 
-void WebUILoginDisplay::LoadWallpaper(const std::string& username) {
-  WallpaperManager::Get()->SetUserWallpaperDelayed(username);
+void WebUILoginDisplay::LoadWallpaper(const user_manager::UserID& user_id) {
+  WallpaperManager::Get()->SetUserWallpaperDelayed(user_id);
 }
 
 void WebUILoginDisplay::LoadSigninWallpaper() {
   WallpaperManager::Get()->SetDefaultWallpaperDelayed(
-      chromeos::login::kSignInUser);
+      chromeos::login::GetSignInUserID());
 }
 
 void WebUILoginDisplay::OnSigninScreenReady() {
@@ -262,7 +262,7 @@ void WebUILoginDisplay::OnSigninScreenReady() {
     delegate_->OnSigninScreenReady();
 }
 
-void WebUILoginDisplay::RemoveUser(const std::string& user_id) {
+void WebUILoginDisplay::RemoveUser(const user_manager::UserID& user_id) {
   SignInScreenController::Get()->RemoveUser(user_id);
 }
 
@@ -304,10 +304,10 @@ void WebUILoginDisplay::SetWebUIHandler(
 }
 
 void WebUILoginDisplay::ShowSigninScreenForCreds(
-    const std::string& username,
+    const user_manager::UserID& user_id,
     const std::string& password) {
   if (webui_handler_)
-    webui_handler_->ShowSigninScreenForCreds(username, password);
+    webui_handler_->ShowSigninScreenForCreds(user_id, password);
 }
 
 bool WebUILoginDisplay::IsShowGuest() const {