Introduce NonElementParentNode

This is the interface where Document.getElementById() belong.

components/script/dom/webidls/NonElementParentNode.webidl

@@ -0,0 +1,11 @@
+/* -*- Mode: IDL; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// https://dom.spec.whatwg.org/#nonelementparentnode
+[NoInterfaceObject,
+ Exposed=Window]
+interface NonElementParentNode {
+  Element? getElementById(DOMString elementId);
+};