Add const versions of some toHTMLFooElement().

Source/web/WebPageSerializerImpl.cpp

 /*
  * Copyright (C) 2009 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 117 matching lines @@
     const Element* element, SerializeDomParam* param, bool* needSkip)
 {
     StringBuilder result;
 
     *needSkip = false;
     if (param->isHTMLDocument) {
         // Skip the open tag of original META tag which declare charset since we
         // have overrided the META which have correct charset declaration after
         // serializing open tag of HEAD element.
         if (element->hasTagName(HTMLNames::metaTag)) {
-            const HTMLMetaElement* meta = static_cast<const HTMLMetaElement*>(element);
+            const HTMLMetaElement* meta = toHTMLMetaElement(element);
             // Check whether the META tag has declared charset or not.
             String equiv = meta->httpEquiv();
             if (equalIgnoringCase(equiv, "content-type")) {
                 String content = meta->content();
                 if (content.length() && content.contains("charset", false)) {
                     // Find META tag declared charset, we need to skip it when
                     // serializing DOM.
                     param->skipMetaElement = element;
                     *needSkip = true;
                 }
@@ skipping 371 matching lines @@
 
         encodeAndFlushBuffer(WebPageSerializerClient::CurrentFrameIsFinished, &param, ForceFlush);
     }
 
     ASSERT(m_dataBuffer.isEmpty());
     m_client->didSerializeDataForFrame(KURL(), WebCString("", 0), WebPageSerializerClient::AllFramesAreFinished);
     return didSerialization;
 }
 
 }  // namespace WebKit