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

Source/core/fetch/Resource.h

Index: Source/core/fetch/Resource.h
diff --git a/Source/core/fetch/Resource.h b/Source/core/fetch/Resource.h
index 7bf7326e0742cee313914c41eb43d97965f7cf59..7db3893879b678071c20e1b67bd8f981142dc8ba 100644
--- a/Source/core/fetch/Resource.h
+++ b/Source/core/fetch/Resource.h
@@ -104,7 +104,7 @@ public:
     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; }