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

Source/modules/serviceworkers/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/#fetchbodystream
+// http://fetch.spec.whatwg.org/#body
 [
     RuntimeEnabled=ServiceWorker,
     Exposed=ServiceWorker,
     NoInterfaceObject,
     ActiveDOMObject,
     GarbageCollected,
-] interface FetchBodyStream {
-    [CallWith=ScriptState] Promise asArrayBuffer();
-    [CallWith=ScriptState] Promise asBlob();
-    [CallWith=ScriptState] Promise asJSON();
-    [CallWith=ScriptState] Promise asText();
+] interface Body {
+    readonly attribute boolean bodyUsed;
+    [CallWith=ScriptState] Promise arrayBuffer();
+    [CallWith=ScriptState] Promise blob();
+    [CallWith=ScriptState] Promise json();
+    [CallWith=ScriptState] Promise text();
 
     // Still to be implemented.
-    // [CallWith=ScriptState] Promise asFormData();
+    // [CallWith=ScriptState] Promise formData();
 };
-