Add missing null check in PageSerializer::serializeCSSStyleSheet()

Source/core/page/PageSerializer.h

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 55 matching lines @@
     // vector is the top frame serialized content.
     void serialize(Page*);
 
     KURL urlForBlankFrame(LocalFrame*);
 
 private:
     void serializeFrame(LocalFrame*);
 
     // Serializes the stylesheet back to text and adds it to the resources if URL is not-empty.
     // It also adds any resources included in that stylesheet (including any imported stylesheets and their own resources).
-    void serializeCSSStyleSheet(CSSStyleSheet*, const KURL&);
+    void serializeCSSStyleSheet(CSSStyleSheet&, const KURL&);
 
     bool shouldAddURL(const KURL&);
 
     void addToResources(Resource *, PassRefPtr<SharedBuffer>, const KURL&);
     void addImageToResources(ImageResource*, RenderObject*, const KURL&);
     void addFontToResources(FontResource*);
 
     void retrieveResourcesForProperties(const StylePropertySet*, Document&);
     void retrieveResourcesForCSSValue(CSSValue*, Document&);
 
     Vector<SerializedResource>* m_resources;
     ListHashSet<KURL> m_resourceURLs;
     HashMap<LocalFrame*, KURL> m_blankFrameURLs;
     unsigned m_blankFrameCounter;
 };
 
 }
 
 #endif