[Fetch API] Implement Request.formData and Response.formData.

third_party/WebKit/Source/modules/fetch/Body.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // https://fetch.spec.whatwg.org/#body
 
 [
     ActiveScriptWrappable,
     DependentLifetime,
     Exposed=ServiceWorker,
     NoInterfaceObject,
 ] interface Body {
     readonly attribute boolean bodyUsed;
     [CallWith=ScriptState] Promise<ArrayBuffer> arrayBuffer();
     [CallWith=ScriptState] Promise<Blob> blob();
+    [CallWith=ScriptState] Promise<FormData> formData();
     [CallWith=ScriptState] Promise<JSON> json();
     [CallWith=ScriptState] Promise<USVString> text();
 
     // body attribute is defined in sub-interfaces, because the IDL processor
     // cannot deal with attribute inheritance with runtime enabled flag.
     // [RuntimeEnabled=ExperimentalStream] readonly attribute ReadableByteStream body;
-
-    // Still to be implemented.
-    // [CallWith=ScriptState] Promise<FormData> formData();
 };