Use MountGuest to mount cryptohome for a demo user.

chrome/browser/chromeos/app_mode/kiosk_profile_loader.cc

Index: chrome/browser/chromeos/app_mode/kiosk_profile_loader.cc
diff --git a/chrome/browser/chromeos/app_mode/kiosk_profile_loader.cc b/chrome/browser/chromeos/app_mode/kiosk_profile_loader.cc
index d1de8428026d6208918deb7b923dc5fdda2ab88f..619c3c0eb1a9aef1dfa1084b5cb1957bd16266b4 100644
--- a/chrome/browser/chromeos/app_mode/kiosk_profile_loader.cc
+++ b/chrome/browser/chromeos/app_mode/kiosk_profile_loader.cc
@@ -10,6 +10,7 @@
 #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"
@@ -149,7 +150,10 @@ void KioskProfileLoader::OnLoginSuccess(const UserContext& user_context)  {
   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