Use Node.isDocumentTypeNode more

Source/core/editing/markup.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009, 2010, 2011 Google Inc. All rights reserved.
  * Copyright (C) 2011 Igalia S.L.
  * Copyright (C) 2011 Motorola Mobility. 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
@@ skipping 868 matching lines @@
     if (!node)
         return String();
 
     Frame* frame = node->document().frame();
     if (!frame)
         return String();
 
     // FIXME: This is never "for interchange". Is that right?
     String markupString = createMarkup(node, IncludeNode, 0);
     Node::NodeType nodeType = node->nodeType();
-    if (nodeType != Node::DOCUMENT_NODE && nodeType != Node::DOCUMENT_TYPE_NODE)
+    if (nodeType != Node::DOCUMENT_NODE && !node->isDocumentTypeNode())
         markupString = frame->documentTypeString() + markupString;
 
     return markupString;
 }
 
 String urlToMarkup(const KURL& url, const String& title)
 {
     StringBuilder markup;
     markup.appendLiteral("<a href=\"");
     markup.append(url.string());
@@ skipping 143 matching lines @@
         return;
 
     RefPtr<Text> textNode = toText(node.get());
     RefPtr<Text> textNext = toText(next);
     textNode->appendData(textNext->data());
     if (textNext->parentNode()) // Might have been removed by mutation event.
         textNext->remove(exceptionState);
 }
 
 }