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

chrome/browser/chromeos/login/auth/chrome_login_performer.cc

Index: chrome/browser/chromeos/login/auth/chrome_login_performer.cc
diff --git a/chrome/browser/chromeos/login/auth/chrome_login_performer.cc b/chrome/browser/chromeos/login/auth/chrome_login_performer.cc
index f63b4d36a79f16cbfb584ac7bdcf0969ed3be241..b42e6b8fa9054c5b34c48f183abb38e86daec987 100644
--- a/chrome/browser/chromeos/login/auth/chrome_login_performer.cc
+++ b/chrome/browser/chromeos/login/auth/chrome_login_performer.cc
@@ -86,13 +86,13 @@ void ChromeLoginPerformer::DidRunTrustedCheck(const base::Closure& callback) {
   }
 }
 
-bool ChromeLoginPerformer::IsUserWhitelisted(const std::string& user_id,
+bool ChromeLoginPerformer::IsUserWhitelisted(const user_manager::UserID& user_id,
                                              bool* wildcard_match) {
   return CrosSettings::IsWhitelisted(user_id, wildcard_match);
 }
 
 void ChromeLoginPerformer::RunOnlineWhitelistCheck(
-    const std::string& user_id,
+    const user_manager::UserID& user_id,
     bool wildcard_match,
     const std::string& refresh_token,
     const base::Closure& success_callback,
@@ -144,18 +144,18 @@ UserContext ChromeLoginPerformer::TransformSupervisedKey(
   return authentication->TransformKey(context);
 }
 
-void ChromeLoginPerformer::SetupSupervisedUserFlow(const std::string& user_id) {
+void ChromeLoginPerformer::SetupSupervisedUserFlow(const user_manager::UserID& user_id) {
   SupervisedUserLoginFlow* new_flow = new SupervisedUserLoginFlow(user_id);
   new_flow->SetHost(ChromeUserManager::Get()->GetUserFlow(user_id)->host());
   ChromeUserManager::Get()->SetUserFlow(user_id, new_flow);
 }
 
-void ChromeLoginPerformer::SetupEasyUnlockUserFlow(const std::string& user_id) {
+void ChromeLoginPerformer::SetupEasyUnlockUserFlow(const user_manager::UserID& user_id) {
   ChromeUserManager::Get()->SetUserFlow(user_id,
                                         new EasyUnlockUserLoginFlow(user_id));
 }
 
-bool ChromeLoginPerformer::CheckPolicyForUser(const std::string& user_id) {
+bool ChromeLoginPerformer::CheckPolicyForUser(const user_manager::UserID& user_id) {
   // Login is not allowed if policy could not be loaded for the account.
   policy::BrowserPolicyConnectorChromeOS* connector =
       g_browser_process->platform_part()->browser_policy_connector_chromeos();