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

chromeos/login/auth/login_performer.cc

Index: chromeos/login/auth/login_performer.cc
diff --git a/chromeos/login/auth/login_performer.cc b/chromeos/login/auth/login_performer.cc
index a48bb434096c49dcf034fec23414db9591ed2a8c..be58ab07a80ba869de3aa2866a64cc2824915a0c 100644
--- a/chromeos/login/auth/login_performer.cc
+++ b/chromeos/login/auth/login_performer.cc
@@ -106,7 +106,7 @@ void LoginPerformer::OnPasswordChangeDetected() {
   }
 }
 
-void LoginPerformer::OnChecked(const std::string& user_id, bool success) {
+void LoginPerformer::OnChecked(const user_manager::UserID& user_id, bool success) {
   if (!delegate_) {
     // Delegate is reset in case of successful offline login.
     // See ExistingUserConstoller::OnAuthSuccess().
@@ -143,10 +143,9 @@ void LoginPerformer::PerformLogin(const UserContext& user_context,
 
 void LoginPerformer::DoPerformLogin(const UserContext& user_context,
                                     AuthorizationMode auth_mode) {
-  std::string email = gaia::CanonicalizeEmail(user_context.GetUserID());
   bool wildcard_match = false;
 
-  if (!IsUserWhitelisted(email, &wildcard_match)) {
+  if (!IsUserWhitelisted(user_context.GetUserID(), &wildcard_match)) {
     NotifyWhitelistCheckFailure();
     return;
   }
@@ -157,7 +156,7 @@ void LoginPerformer::DoPerformLogin(const UserContext& user_context,
   switch (auth_mode_) {
     case AUTH_MODE_EXTENSION: {
       RunOnlineWhitelistCheck(
-          email, wildcard_match, user_context.GetRefreshToken(),
+          user_context.GetUserID(), wildcard_match, user_context.GetRefreshToken(),
           base::Bind(&LoginPerformer::StartLoginCompletion,
                      weak_factory_.GetWeakPtr()),
           base::Bind(&LoginPerformer::NotifyWhitelistCheckFailure,
@@ -172,7 +171,7 @@ void LoginPerformer::DoPerformLogin(const UserContext& user_context,
 
 void LoginPerformer::LoginAsSupervisedUser(const UserContext& user_context) {
   DCHECK_EQ(chromeos::login::kSupervisedUserDomain,
-            gaia::ExtractDomainName(user_context.GetUserID()));
+            gaia::ExtractDomainName(user_context.GetUserID().GetUserEmail()));
 
   user_context_ = user_context;
   user_context_.SetUserType(user_manager::USER_TYPE_SUPERVISED);
@@ -238,7 +237,7 @@ void LoginPerformer::LoginOffTheRecord() {
       base::Bind(&Authenticator::LoginOffTheRecord, authenticator_.get()));
 }
 
-void LoginPerformer::LoginAsKioskAccount(const std::string& app_user_id,
+void LoginPerformer::LoginAsKioskAccount(const user_manager::UserID& app_user_id,
                                          bool use_guest_mount) {
   EnsureAuthenticator();
   task_runner_->PostTask(FROM_HERE,