[Fetch API] Put body members directly on Response/Request

Source/modules/serviceworkers/Response.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/#response-class
 
 enum ResponseType { "basic", "cors", "default", "error", "opaque" };
 
 [
     // FIXME: Add ctors for ArrayBuffer, ArrayBufferView, FormData,
     // and URLSearchParams response bodies.
     Constructor(ScalarValueString body, optional Dictionary responseInitDict),
     Constructor(Blob? body, optional Dictionary responseInitDict),
+    ConstructorCallWith=ExecutionContext,
     RuntimeEnabled=ServiceWorker,
     Exposed=ServiceWorker,
     RaisesException=Constructor,
     GarbageCollected,
 ] interface Response {
     // FIXME: Implement redirect().
     readonly attribute ResponseType type;
     readonly attribute ScalarValueString url;
     readonly attribute unsigned short status;
     readonly attribute ByteString statusText;
     readonly attribute Headers headers;
-    [CallWith=ExecutionContext] readonly attribute FetchBodyStream body;    
 };
+
+Response implements Body;