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

Source/core/fetch/Resource.h

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller <mueller@kde.org>
     Copyright (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
     Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later version.
@@ skipping 86 matching lines @@
 
     void setResourceError(const ResourceError& error) { m_error = error; }
     const ResourceError& resourceError() const { return m_error; }
 
     void setIdentifier(unsigned long identifier) { m_identifier = identifier; }
     unsigned long identifier() const { return m_identifier; }
 
     virtual bool shouldIgnoreHTTPStatusCodeErrors() const { return false; }
 
     ResourceRequest& resourceRequest() { return m_resourceRequest; }
-    const ResourceRequest& lastResourceRequest();
+    const ResourceRequest& lastResourceRequest() const;
     const KURL& url() const { return m_resourceRequest.url();}
     Type type() const { return static_cast<Type>(m_type); }
     const ResourceLoaderOptions& options() const { return m_options; }
     void setOptions(const ResourceLoaderOptions& options) { m_options = options; }
 
     void didChangePriority(ResourceLoadPriority, int intraPriorityValue);
 
     void addClient(ResourceClient*);
     void removeClient(ResourceClient*);
     bool hasClients() const { return !m_clients.isEmpty() || !m_clientsAwaitingCallback.isEmpty(); }
@@ skipping 277 matching lines @@
 const char* ResourceTypeName(Resource::Type);
 #endif
 
 #define DEFINE_RESOURCE_TYPE_CASTS(typeName) \
     DEFINE_TYPE_CASTS(typeName##Resource, Resource, resource, resource->type() == Resource::typeName, resource.type() == Resource::typeName); \
     inline typeName##Resource* to##typeName##Resource(const ResourcePtr<Resource>& ptr) { return to##typeName##Resource(ptr.get()); }
 
 }
 
 #endif