implement HTMLOptionsCollection and add related items to HTMLSelectElement

components/script/dom/webidls/HTMLOptionsCollection.webidl

@@ -0,0 +1,19 @@
+/* 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/#htmloptionscollection
+[Exposed=(Window,Worker)]
+interface HTMLOptionsCollection : HTMLCollection {
+  // inherits item(), namedItem()
+  attribute unsigned long length; // shadows inherited length
+  //[CEReactions]
+  [Throws]
+  setter void (unsigned long index, HTMLOptionElement? option);
+  //[CEReactions]
+  [Throws]
+  void add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
+  //[CEReactions]
+  void remove(long index);
+  //attribute long selectedIndex;
+};