add webidl bindings for DOMStringList

components/script/dom/webidls/DOMStringList.webidl

@@ -0,0 +1,18 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+/*
+ * The origin of this IDL file is
+ * https://html.spec.whatwg.org/multipage/#domstringlist
+ *
+ * Copyright:
+ * To the extent possible under law, the editors have waived all copyright and
+ * related or neighboring rights to this work.
+ */
+
+[Exposed=(Window,Worker)]
+interface DOMStringList {
+  readonly attribute unsigned long length;
+  getter DOMString? item(unsigned long index);
+  boolean contains(DOMString string);
+};