ChromeOS cryptohome should be able to use gaia id as user identifier.

chrome/browser/chromeos/app_mode/kiosk_app_manager.h

 // 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.
 
 #ifndef CHROME_BROWSER_CHROMEOS_APP_MODE_KIOSK_APP_MANAGER_H_
 #define CHROME_BROWSER_CHROMEOS_APP_MODE_KIOSK_APP_MANAGER_H_
 
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "base/callback_forward.h"
 #include "base/lazy_instance.h"
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/scoped_vector.h"
 #include "base/observer_list.h"
 #include "base/time/time.h"
 #include "chrome/browser/chromeos/app_mode/kiosk_app_data_delegate.h"
 #include "chrome/browser/chromeos/extensions/external_cache.h"
 #include "chrome/browser/chromeos/policy/enterprise_install_attributes.h"
 #include "chrome/browser/chromeos/settings/cros_settings.h"
+#include "components/signin/core/account_id/account_id.h"
 #include "ui/gfx/image/image_skia.h"
 
 class PrefRegistrySimple;
 class Profile;
 
 namespace base {
 class RefCountedString;
 }
 
 namespace extensions {
@@ skipping 30 matching lines @@
 
   // Struct to hold app info returned from GetApps() call.
   struct App {
     App(const KioskAppData& data,
         bool is_extension_pending,
         bool was_auto_launched_with_zero_delay);
     App();
     ~App();
 
     std::string app_id;
-    std::string user_id;
+    AccountId account_id;
     std::string name;
     gfx::ImageSkia icon;
     std::string required_platform_version;
     bool is_loading;
     bool was_auto_launched_with_zero_delay;
   };
   typedef std::vector<App> Apps;
 
   // Name of a dictionary that holds kiosk app info in Local State.
   // Sample layout:
@@ skipping 239 matching lines @@
   base::WeakPtr<KioskAppExternalLoader> secondary_app_external_loader_;
 
   scoped_ptr<AppSession> app_session_;
 
   DISALLOW_COPY_AND_ASSIGN(KioskAppManager);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_APP_MODE_KIOSK_APP_MANAGER_H_