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

chrome/browser/chromeos/login/signin_screen_controller.cc

Index: chrome/browser/chromeos/login/signin_screen_controller.cc
diff --git a/chrome/browser/chromeos/login/signin_screen_controller.cc b/chrome/browser/chromeos/login/signin_screen_controller.cc
index 40f73796e46587ec8fe431061ab7c8be3b32c35d..b2533042de0e7c80a24e4361c04982b8c1972224 100644
--- a/chrome/browser/chromeos/login/signin_screen_controller.cc
+++ b/chrome/browser/chromeos/login/signin_screen_controller.cc
@@ -58,16 +58,16 @@ void SignInScreenController::OnSigninScreenReady() {
   }
 }
 
-void SignInScreenController::RemoveUser(const std::string& user_id) {
+void SignInScreenController::RemoveUser(const user_manager::UserID& user_id) {
   user_manager::UserManager::Get()->RemoveUser(user_id, this);
 }
 
-void SignInScreenController::OnBeforeUserRemoved(const std::string& username) {
-  user_selection_screen_->OnBeforeUserRemoved(username);
+void SignInScreenController::OnBeforeUserRemoved(const user_manager::UserID& user_id) {
+  user_selection_screen_->OnBeforeUserRemoved(user_id);
 }
 
-void SignInScreenController::OnUserRemoved(const std::string& username) {
-  user_selection_screen_->OnUserRemoved(username);
+void SignInScreenController::OnUserRemoved(const user_manager::UserID& user_id) {
+  user_selection_screen_->OnUserRemoved(user_id);
 }
 
 void SignInScreenController::SendUserList() {
@@ -78,7 +78,7 @@ const user_manager::UserList& SignInScreenController::GetUsers() {
   return user_selection_screen_->GetUsers();
 }
 
-void SignInScreenController::CheckUserStatus(const std::string& user_id) {
+void SignInScreenController::CheckUserStatus(const user_manager::UserID& user_id) {
   user_selection_screen_->CheckUserStatus(user_id);
 }