[ServiceWorker] Make fetch() method better conformance with the spec.

Source/modules/serviceworkers/Response.cpp

 // 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.
 
 #include "config.h"
 #include "Response.h"
 
 #include "bindings/core/v8/Dictionary.h"
 #include "core/fileapi/Blob.h"
 #include "modules/serviceworkers/FetchBodyStream.h"
@@ skipping 135 matching lines @@
 }
 
 PassRefPtrWillBeRawPtr<Headers> Response::headers() const
 {
     // "The headers attribute's getter must return the associated Headers object."
     return m_headers;
 }
 
 PassRefPtrWillBeRawPtr<FetchBodyStream> Response::body(ExecutionContext* context)
 {
+    if (!m_response->blobDataHandle())
+        return nullptr;
     if (!m_fetchBodyStream)
         m_fetchBodyStream = FetchBodyStream::create(context, m_response->blobDataHandle());
     return m_fetchBodyStream;
 }
 
 void Response::populateWebServiceWorkerResponse(blink::WebServiceWorkerResponse& response)
 {
     m_response->populateWebServiceWorkerResponse(response);
 }
 
@@ skipping 14 matching lines @@
 }
 
 void Response::trace(Visitor* visitor)
 {
     visitor->trace(m_response);
     visitor->trace(m_headers);
     visitor->trace(m_fetchBodyStream);
 }
 
 } // namespace blink