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

chromeos/login/auth/stub_authenticator.h

Index: chromeos/login/auth/stub_authenticator.h
diff --git a/chromeos/login/auth/stub_authenticator.h b/chromeos/login/auth/stub_authenticator.h
index 8c67874abe2aa665f7def88a9f778dbebd30ccc0..10f9775296b0e4c4e784d65838bbc687e269b243 100644
--- a/chromeos/login/auth/stub_authenticator.h
+++ b/chromeos/login/auth/stub_authenticator.h
@@ -13,6 +13,8 @@
 #include "chromeos/login/auth/authenticator.h"
 #include "chromeos/login/auth/user_context.h"
 
+class AccountId;
+
 namespace content {
 class BrowserContext;
 }
@@ -35,7 +37,7 @@ class CHROMEOS_EXPORT StubAuthenticator : public Authenticator {
   void LoginAsSupervisedUser(const UserContext& user_context) override;
   void LoginOffTheRecord() override;
   void LoginAsPublicSession(const UserContext& user_context) override;
-  void LoginAsKioskAccount(const std::string& app_user_id,
+  void LoginAsKioskAccount(const AccountId& app_account_id,
                            bool use_guest_mount) override;
   void OnAuthSuccess() override;
   void OnAuthFailure(const AuthFailure& failure) override;