Replace static_cast with safe casting methods

Source/core/page/PageSerializer.cpp

 /*
  * 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 311 matching lines @@
 
     // The background-image and list-style-image (for ul or ol) are the CSS properties
     // that make use of images. We iterate to make sure we include any other
     // image properties there might be.
     unsigned propertyCount = styleDeclaration->propertyCount();
     for (unsigned i = 0; i < propertyCount; ++i) {
         RefPtr<CSSValue> cssValue = styleDeclaration->propertyAt(i).value();
         if (!cssValue->isImageValue())
             continue;
 
-        CSSImageValue* imageValue = static_cast<CSSImageValue*>(cssValue.get());
+        CSSImageValue* imageValue = toCSSImageValue(cssValue.get());
         StyleImage* styleImage = imageValue->cachedOrPendingImage();
         // Non cached-images are just place-holders and do not contain data.
         if (!styleImage || !styleImage->isCachedImage())
             continue;
 
         CachedImage* image = static_cast<StyleCachedImage*>(styleImage)->cachedImage();
         addImageToResources(image, 0, image->url());
     }
 }
 
 KURL PageSerializer::urlForBlankFrame(Frame* frame)
 {
     HashMap<Frame*, KURL>::iterator iter = m_blankFrameURLs.find(frame);
     if (iter != m_blankFrameURLs.end())
         return iter->value;
     String url = "wyciwyg://frame/" + String::number(m_blankFrameCounter++);
     KURL fakeURL(ParsedURLString, url);
     m_blankFrameURLs.add(frame, fakeURL);
 
     return fakeURL;
 }
 
 }