ServiceWorker: Add a Response ctor that accepts the body as a Blob

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.
 
 // https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#response-objects
 // FIXME: Split this idl/impl into AbstractResponse and Response.
 [
+    // FIXME: Remove this legacy function after the required Chromium-side patch lands.
     Constructor(optional Dictionary responseInitDict),
+    // FIXME: Add ctors for String, Stream, and ArrayBuffer/ArrayBufferView response bodies.
+    Constructor(Blob body, optional Dictionary responseInitDict),
     RuntimeEnabled=ServiceWorker,
     Exposed=ServiceWorker
 ] interface Response {
     readonly attribute unsigned short status;
 
     // FIXME: Spec uses ByteString for this. We must perform the DOMString -> ByteString conversion manually (crbug.com/347426).
     readonly attribute DOMString statusText;
 
     readonly attribute HeaderMap headers;
 
     // FIXME: Implement the following:
     // attribute DOMString url;
     // Promise<Blob> toBlob();
 };