[ServiceWorker] Make Response class better conformance with the spec.

Source/modules/serviceworkers/HeaderMap.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/#headers-class
-// FIXME: Fetch spec has renamed this to just 'Headers'
-// FIXME: This should be defined as MapClass(DOMString, DOMString)
-// once binding code supports it (http://crbug.com/347435).
-[
-    Constructor,
-    RuntimeEnabled=ServiceWorker,
-    Exposed=ServiceWorker
-] interface HeaderMap {
-    readonly attribute unsigned long size;
-
-    // FIXME: Define keys, values, entries.
-    // FIXME: Some of the method definitions (like set) don't strictly
-    // follow the MapClass's ones (http://heycam.github.io/webidl/#MapClass),
-    // as this is supposed to be a transient implementation until we have real
-    // MapClass.
-
-    void clear();
-    [ImplementedAs=remove] boolean delete(DOMString key);
-    DOMString get(DOMString key);
-    boolean has(DOMString key);
-    void set(DOMString key, DOMString value);
-    void forEach(HeaderMapForEachCallback callback, optional any thisArg);
-};