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

chrome/browser/chromeos/app_mode/kiosk_profile_loader.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_PROFILE_LOADER_H_
 #define CHROME_BROWSER_CHROMEOS_APP_MODE_KIOSK_PROFILE_LOADER_H_
 
 #include <string>
 
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "chrome/browser/chromeos/app_mode/kiosk_app_launch_error.h"
 #include "chrome/browser/chromeos/login/session/user_session_manager.h"
 #include "chromeos/login/auth/login_performer.h"
+#include "components/signin/core/account_id/account_id.h"
 
 class Profile;
 
 namespace chromeos {
 
 // KioskProfileLoader loads a special profile for a given app. It first
 // attempts to login for the app's generated user id. If the login is
 // successful, it prepares app profile then calls the delegate.
 class KioskProfileLoader : public LoginPerformer::Delegate,
                            public UserSessionManagerDelegate {
  public:
   class Delegate {
    public:
     virtual void OnProfileLoaded(Profile* profile) = 0;
     virtual void OnProfileLoadFailed(KioskAppLaunchError::Error error) = 0;
 
    protected:
     virtual ~Delegate() {}
   };
 
-  KioskProfileLoader(const std::string& app_user_id,
+  KioskProfileLoader(const AccountId& app_account_id,
                      bool use_guest_mount,
                      Delegate* delegate);
 
   ~KioskProfileLoader() override;
 
   // Starts profile load. Calls delegate on success or failure.
   void Start();
 
  private:
   class CryptohomedChecker;
 
   void LoginAsKioskAccount();
   void ReportLaunchResult(KioskAppLaunchError::Error error);
 
   // LoginPerformer::Delegate overrides:
   void OnAuthSuccess(const UserContext& user_context) override;
   void OnAuthFailure(const AuthFailure& error) override;
   void WhiteListCheckFailed(const std::string& email) override;
   void PolicyLoadFailed() override;
   void SetAuthFlowOffline(bool offline) override;
 
   // UserSessionManagerDelegate implementation:
   void OnProfilePrepared(Profile* profile, bool browser_launched) override;
 
-  std::string user_id_;
+  const AccountId account_id_;
   bool use_guest_mount_;
   Delegate* delegate_;
   scoped_ptr<CryptohomedChecker> cryptohomed_checker_;
   scoped_ptr<LoginPerformer> login_performer_;
 
   DISALLOW_COPY_AND_ASSIGN(KioskProfileLoader);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_APP_MODE_KIOSK_PROFILE_LOADER_H_