Remove useless null checks from Node::document() call sites

Source/core/xml/XMLSerializer.cpp

 /*
  *  Copyright (C) 2003, 2006 Apple Inc. All rights reserved.
  *  Copyright (C) 2006 Samuel Weinig (sam@webkit.org)
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
  *  version 2 of the License, or (at your option) any later version.
  *
  *  This library is distributed in the hope that it will be useful,
@@ skipping 15 matching lines @@
 #include "core/editing/markup.h"
 #include "wtf/text/WTFString.h"
 
 namespace WebCore {
 
 String XMLSerializer::serializeToString(Node* node, ExceptionState& es)
 {
     if (!node)
         return String();
 
-    if (!node->document()) {
-        // Due to the fact that DocumentType nodes are created by the DOMImplementation
-        // and not the Document, it is possible for it to not have a Document associated
-        // with it.  It should be the only type of node where this is possible.
-        ASSERT(node->nodeType() == Node::DOCUMENT_TYPE_NODE);
-
-        es.throwDOMException(InvalidAccessError);
-        return String();
-    }
-
     return createMarkup(node);
 }
 
 } // namespace WebCore