Add WebIDL support and two sample file

samples/WebIDL/Fetch.webidl

@@ -0,0 +1,35 @@
+/* -*- 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/.
+ *
+ * The origin of this IDL file is
+ * http://fetch.spec.whatwg.org/
+ */
+
+typedef object JSON;
+// FIXME(nsm): Bug 739173: FormData is not available in workers.
+// typedef (ArrayBuffer or ArrayBufferView or Blob or FormData or USVString or URLSearchParams) BodyInit;
+typedef (ArrayBuffer or ArrayBufferView or Blob or USVString or URLSearchParams) BodyInit;
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface Body {
+  readonly attribute boolean bodyUsed;
+  [Throws]
+  Promise<ArrayBuffer> arrayBuffer();
+  [Throws]
+  Promise<Blob> blob();
+  // FIXME(nsm): Bug 739173 FormData is not supported in workers.
+  // Promise<FormData> formData();
+  [Throws]
+  Promise<JSON> json();
+  [Throws]
+  Promise<USVString> text();
+};
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface GlobalFetch {
+  [Throws, Func="mozilla::dom::Headers::PrefEnabled"]
+  Promise<Response> fetch(RequestInfo input, optional RequestInit init);
+};
+