Devirtualize Document class type checking

Source/core/html/PluginDocument.cpp

 /*
  * Copyright (C) 2006, 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 118 matching lines @@
             frame->loader()->client()->redirectDataToPlugin(widget);
             // In a plugin document, the main resource is the plugin. If we have a null widget, that means
             // the loading of the plugin was cancelled, which gives us a null mainResourceLoader(), so we
             // need to have this call in a null check of the widget or of mainResourceLoader().
             frame->loader()->activeDocumentLoader()->setMainResourceDataBufferingPolicy(DoNotBufferData);
         }
     }
 }
 
 PluginDocument::PluginDocument(Frame* frame, const KURL& url)
-    : HTMLDocument(frame, url)
+    : HTMLDocument(frame, url, PluginDocumentClass)
     , m_shouldLoadPluginManually(true)
 {
     setCompatibilityMode(QuirksMode);
     lockCompatibilityMode();
 }
 
 PassRefPtr<DocumentParser> PluginDocument::createParser()
 {
     return PluginDocumentParser::create(this);
 }
@@ skipping 27 matching lines @@
     // with how many times we call beforeload on object elements. <rdar://problem/8441094>.
     if (!shouldLoadPluginManually())
         return;
 
     DocumentLoader* documentLoader = frame()->loader()->activeDocumentLoader();
     documentLoader->cancelMainResourceLoad(frame()->loader()->cancelledError(documentLoader->request()));
     setShouldLoadPluginManually(false);
 }
 
 }