Support value iterators in WebIDL interfaces.

components/script/dom/webidls/TestBindingIterable.webidl

@@ -0,0 +1,14 @@
+/* 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/. */
+
+// This interface is entirely internal to Servo, and should not be accessible to
+// web pages.
+
+[Pref="dom.testbinding.enabled", Exposed=(Window,Worker), Constructor]
+interface TestBindingIterable {
+  void add(DOMString arg);
+  readonly attribute unsigned long length;
+  getter DOMString getItem(unsigned long index);
+  iterable<DOMString>;
+};