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

chrome/browser/chromeos/login/existing_user_controller.h

Index: chrome/browser/chromeos/login/existing_user_controller.h
diff --git a/chrome/browser/chromeos/login/existing_user_controller.h b/chrome/browser/chromeos/login/existing_user_controller.h
index b5e4d0f480a0510315d6ee68582c0c5eabfe62f8..c5710f2bb7d8cf3ab3b2657cd3f731917a78debd 100644
--- a/chrome/browser/chromeos/login/existing_user_controller.h
+++ b/chrome/browser/chromeos/login/existing_user_controller.h
@@ -25,6 +25,7 @@
 #include "chrome/browser/chromeos/settings/device_settings_service.h"
 #include "chromeos/login/auth/login_performer.h"
 #include "chromeos/login/auth/user_context.h"
+#include "components/signin/core/account_id/account_id.h"
 #include "components/user_manager/user.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
@@ -247,8 +248,8 @@ class ExistingUserController : public LoginDisplay::Delegate,
   // Public session auto-login timeout, in milliseconds.
   int public_session_auto_login_delay_;
 
-  // Username for public session auto-login.
-  std::string public_session_auto_login_username_;
+  // AccountId for public session auto-login.
+  AccountId public_session_auto_login_account_id_ = EmptyAccountId();
 
   // Used to execute login operations.
   scoped_ptr<LoginPerformer> login_performer_;