Add a mojo interface for AccountId

components/signin/public/interfaces/account_id.mojom

Index: components/signin/public/interfaces/account_id.mojom
diff --git a/components/signin/public/interfaces/account_id.mojom b/components/signin/public/interfaces/account_id.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..26edd754fc0d6c527ce5ece75ded29d8e8db9630
--- /dev/null
+++ b/components/signin/public/interfaces/account_id.mojom
@@ -0,0 +1,10 @@
+// Copyright 2017 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.
+
+module signin.mojom;
+
+// Represents AccountId in Mojo.
+struct AccountId {
+  string serialized;
+};