Move Fetch API releted code to modules/fetch.

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.
 
 // http://fetch.spec.whatwg.org/#body
 typedef object JSON;
 [
     RuntimeEnabled=ServiceWorker,
     Exposed=ServiceWorker,
     NoInterfaceObject,
     ActiveDOMObject,
     GarbageCollected,
     TypeChecking=Interface,
 ] interface Body {
     readonly attribute boolean bodyUsed;
     [CallWith=ScriptState] Promise<ArrayBuffer> arrayBuffer();
     [CallWith=ScriptState] Promise<Blob> blob();
     [CallWith=ScriptState] Promise<JSON> json();
     [CallWith=ScriptState] Promise<USVString> text();
 
     [RuntimeEnabled=Stream] readonly attribute ReadableStream body;
     // Still to be implemented.
     // [CallWith=ScriptState] Promise<FormData> formData();
 };