Implement the Fetch method

components/script/dom/webidls/Fetch.webidl

@@ -0,0 +1,11 @@
+/* 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://fetch.spec.whatwg.org/#fetch-method
+
+[Exposed=(Window,Worker)]
+
+partial interface WindowOrWorkerGlobalScope {
+  [NewObject] Promise<Response> fetch(RequestInfo input, optional RequestInit init);
+};