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

chromeos/login/auth/utils.cc

Index: chromeos/login/auth/utils.cc
diff --git a/chromeos/login/auth/utils.cc b/chromeos/login/auth/utils.cc
new file mode 100644
index 0000000000000000000000000000000000000000..c36e9bdafed8e90369142eb339ce0adc4566db43
--- /dev/null
+++ b/chromeos/login/auth/utils.cc
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+// Copyright 2016 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.
+
+#include "chromeos/login/auth/utils.h"
+
+#include "chromeos/cryptohome/cryptohome_parameters.h"
+#include "components/signin/core/account_id/account_id.h"
+#include "components/user_manager/known_user.h"
+
+namespace cryptohome {
+
+class AccountId GetAccountId(const Identification& user_id) {
+  const std::vector<AccountId> known_account_ids =
+      user_manager::known_user::GetKnownAccountIds();
+
+  for (const AccountId& known_id : known_account_ids) {
+    if (known_id.GetGaiaIdKey() == user_id.id())
+      return known_id;
+  }
+
+  return user_manager::known_user::GetAccountId(user_id.id(),
+                                                std::string() /* gaia_id */);
+}
+
+}  // namespace cryptohome