Use MountGuest to mount cryptohome for a demo user.

chrome/browser/chromeos/app_mode/kiosk_profile_loader.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/chromeos/app_mode/kiosk_profile_loader.h"
 
 #include "base/logging.h"
 #include "base/memory/weak_ptr.h"
 #include "base/message_loop/message_loop.h"
 #include "base/strings/string_util.h"
 #include "base/sys_info.h"
 #include "chrome/browser/chromeos/app_mode/kiosk_app_manager.h"
+#include "chrome/browser/chromeos/login/demo_mode/demo_app_launcher.h"
 #include "chrome/browser/chromeos/login/login_display_host_impl.h"
 #include "chrome/browser/chromeos/login/login_status_consumer.h"
 #include "chrome/browser/chromeos/login/login_utils.h"
 #include "chrome/browser/chromeos/login/user_manager.h"
 #include "chrome/browser/chromeos/settings/cros_settings.h"
 #include "chrome/browser/lifetime/application_lifetime.h"
 #include "chromeos/cryptohome/async_method_caller.h"
 #include "chromeos/dbus/cryptohome_client.h"
 #include "chromeos/dbus/dbus_thread_manager.h"
 #include "content/public/browser/browser_thread.h"
@@ skipping 119 matching lines @@
   if (error != KioskAppLaunchError::NONE) {
     delegate_->OnProfileLoadFailed(error);
   }
 }
 
 void KioskProfileLoader::OnLoginSuccess(const UserContext& user_context)  {
   // LoginPerformer will delete itself.
   login_performer_->set_delegate(NULL);
   ignore_result(login_performer_.release());
 
-  LoginUtils::Get()->PrepareProfile(user_context,
+  UserContext context = user_context;
+  if (context.username == UserManager::kGuestUserName)

[Dmitry Polukhin, 2014/02/20 00:02:48]

Please add comment why you do this substitution.

[rkc, 2014/02/20 00:05:38]

Done.

+    context.username = DemoAppLauncher::kDemoUserName;
+  LoginUtils::Get()->PrepareProfile(context,
                                     std::string(),  // display email
                                     false,  // has_cookies
                                     false,  // has_active_session
                                     this);
 }
 
 void KioskProfileLoader::OnLoginFailure(const LoginFailure& error) {
   ReportLaunchResult(LoginFailureToKioskAppLaunchError(error));
 }
 
@@ skipping 13 matching lines @@
 void KioskProfileLoader::OnProfilePrepared(Profile* profile) {
   // This object could be deleted any time after successfully reporting
   // a profile load, so invalidate the LoginUtils delegate now.
   LoginUtils::Get()->DelegateDeleted(this);
 
   delegate_->OnProfileLoaded(profile);
   ReportLaunchResult(KioskAppLaunchError::NONE);
 }
 
 }  // namespace chromeos