Use MountGuest to mount cryptohome for a demo user.

chrome/browser/chromeos/login/parallel_authenticator.cc

Index: chrome/browser/chromeos/login/parallel_authenticator.cc
diff --git a/chrome/browser/chromeos/login/parallel_authenticator.cc b/chrome/browser/chromeos/login/parallel_authenticator.cc
index 85ae4b9739f53f3c7e48bcb36fd416e20073f0ec..29ffe01df4da276ab7900e54bca1ab2e094bd509 100644
--- a/chrome/browser/chromeos/login/parallel_authenticator.cc
+++ b/chrome/browser/chromeos/login/parallel_authenticator.cc
@@ -348,19 +348,28 @@ void ParallelAuthenticator::LoginAsPublicAccount(const std::string& username) {
 void ParallelAuthenticator::LoginAsKioskAccount(
     const std::string& app_user_id, bool force_ephemeral) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+
+  std::string user_id =
+      force_ephemeral ? UserManager::kGuestUserName : app_user_id;
   current_state_.reset(new AuthAttemptState(
-      UserContext(app_user_id,
+      UserContext(user_id,
                   std::string(),  // password
                   std::string()),  // auth_code
       std::string(),  // login_token
       std::string(),  // login_captcha
       User::USER_TYPE_KIOSK_APP,
       false));
+
   remove_user_data_on_failure_ = true;
-  int ephemeral_flag = force_ephemeral ? cryptohome::ENSURE_EPHEMERAL : 0;
-  MountPublic(current_state_.get(),
-        scoped_refptr<ParallelAuthenticator>(this),
-        cryptohome::CREATE_IF_MISSING | ephemeral_flag);
+  if (!force_ephemeral) {
+    MountPublic(current_state_.get(),
+          scoped_refptr<ParallelAuthenticator>(this),
+          cryptohome::CREATE_IF_MISSING);
+  } else {
+    ephemeral_mount_attempted_ = true;
+    MountGuest(current_state_.get(),
+               scoped_refptr<ParallelAuthenticator>(this));
+  }
 }
 
 void ParallelAuthenticator::OnRetailModeLoginSuccess() {