Use MountGuest to mount cryptohome for a demo user.

chrome/browser/chromeos/login/demo_mode/demo_app_launcher.cc

Index: chrome/browser/chromeos/login/demo_mode/demo_app_launcher.cc
diff --git a/chrome/browser/chromeos/login/demo_mode/demo_app_launcher.cc b/chrome/browser/chromeos/login/demo_mode/demo_app_launcher.cc
index 6503d9b9708e6e0ddb8323af1301025991e6f7fa..223cc18b8cb5018dc84c902f22aa743d6b43a086 100644
--- a/chrome/browser/chromeos/login/demo_mode/demo_app_launcher.cc
+++ b/chrome/browser/chromeos/login/demo_mode/demo_app_launcher.cc
@@ -15,14 +15,10 @@
 #include "extensions/browser/extension_system.h"
 #include "grit/browser_resources.h"
 
-namespace {
-
-const char kDemoAppUserId[] = "demouser@demo.app.local";
-
-}
-
 namespace chromeos {
 
+const char DemoAppLauncher::kDemoUserName[] = "demouser@demo.app.local";
+
 DemoAppLauncher::DemoAppLauncher() : profile_(NULL) {}
 
 DemoAppLauncher::~DemoAppLauncher() {}
@@ -31,13 +27,13 @@ void DemoAppLauncher::StartDemoAppLaunch() {
   DVLOG(1) << "Launching demo app...";
   // user_id = DemoAppUserId, force_emphemeral = true, delegate = this.
   kiosk_profile_loader_.reset(
-      new KioskProfileLoader(kDemoAppUserId, true, this));
+      new KioskProfileLoader(kDemoUserName, true, this));
   kiosk_profile_loader_->Start();
 }
 
 // static
 bool DemoAppLauncher::IsDemoAppSession(const std::string& user_id) {
-  return user_id == kDemoAppUserId ? true : false;
+  return user_id == kDemoUserName ? true : false;
 }
 
 void DemoAppLauncher::OnProfileLoaded(Profile* profile) {