Devirtualize Document class type checking

Source/core/html/MediaDocument.cpp

 /*
  * Copyright (C) 2008 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 99 matching lines @@
 void MediaDocumentParser::appendBytes(DocumentWriter*, const char*, size_t)
 {
     if (m_mediaElement)
         return;
 
     createDocumentStructure();
     finish();
 }
     
 MediaDocument::MediaDocument(Frame* frame, const KURL& url)
-    : HTMLDocument(frame, url)
+    : HTMLDocument(frame, url, MediaDocumentClass)
     , m_replaceMediaElementTimer(this, &MediaDocument::replaceMediaElementTimerFired)
 {
     setCompatibilityMode(QuirksMode);
     lockCompatibilityMode();
 }
 
 MediaDocument::~MediaDocument()
 {
     ASSERT(!m_replaceMediaElementTimer.isActive());
 }
@@ skipping 83 matching lines @@
         DocumentLoader* documentLoader = loader();
         ASSERT(documentLoader);
         if (documentLoader)
             embedElement->setAttribute(typeAttr, documentLoader->writer()->mimeType());
 
         videoElement->parentNode()->replaceChild(embedElement, videoElement, IGNORE_EXCEPTION);
     }
 }
 
 }