Implement HTMLFormElement::Elements

Fixes #8566

components/script/dom/webidls/HTMLFormControlsCollection.webidl

@@ -0,0 +1,16 @@
+/* -*- 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://html.spec.whatwg.org/multipage/#htmlformcontrolscollection
+interface HTMLFormControlsCollection : HTMLCollection {
+  // inherits length and item()
+  // getter (RadioNodeList or Element)? namedItem(DOMString name); // shadows inherited namedItem()
+};
+
+/*
+interface RadioNodeList : NodeList {
+  attribute DOMString value;
+};
+*/