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

chromeos/login/auth/cryptohome_authenticator.cc

Index: chromeos/login/auth/cryptohome_authenticator.cc
diff --git a/chromeos/login/auth/cryptohome_authenticator.cc b/chromeos/login/auth/cryptohome_authenticator.cc
index 00c79d47ca840227748319950fc58f422d6870b3..069cd4840d909f7a743a91ec107908c7d8497e47 100644
--- a/chromeos/login/auth/cryptohome_authenticator.cc
+++ b/chromeos/login/auth/cryptohome_authenticator.cc
@@ -293,7 +293,7 @@ void MountPublic(const base::WeakPtr<AuthAttemptState>& attempt,
                  scoped_refptr<CryptohomeAuthenticator> resolver,
                  int flags) {
   cryptohome::AsyncMethodCaller::GetInstance()->AsyncMountPublic(
-      attempt->user_context.GetUserID(),
+      attempt->user_context.GetUserID().GetUserEmail(),
       flags,
       base::Bind(&TriggerResolveWithLoginTimeMarker,
                  "CryptohomeMountPublic-End",
@@ -459,7 +459,7 @@ void CryptohomeAuthenticator::LoginOffTheRecord() {
   DCHECK(task_runner_->RunsTasksOnCurrentThread());
   current_state_.reset(
       new AuthAttemptState(UserContext(user_manager::USER_TYPE_GUEST,
-                                       chromeos::login::kGuestUserName),
+                                       chromeos::login::GetGuestUserID()),
                            false,    // unlock
                            false,    // online_complete
                            false));  // user_is_new
@@ -487,12 +487,12 @@ void CryptohomeAuthenticator::LoginAsPublicSession(
 }
 
 void CryptohomeAuthenticator::LoginAsKioskAccount(
-    const std::string& app_user_id,
+    const user_manager::UserID& app_user_id,
     bool use_guest_mount) {
   DCHECK(task_runner_->RunsTasksOnCurrentThread());
 
-  const std::string user_id =
-      use_guest_mount ? chromeos::login::kGuestUserName : app_user_id;
+  const user_manager::UserID user_id =
+      use_guest_mount ? chromeos::login::GetGuestUserID() : app_user_id;
   current_state_.reset(new AuthAttemptState(
       UserContext(user_manager::USER_TYPE_KIOSK_APP, user_id),
       false,    // unlock