Use new is*Element() helper functions even more in HTML code

Source/core/html/HTMLDocument.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 96 matching lines @@
     else if (equalIgnoringCase(value, "off"))
         mode = off;
     else
         mode = inherit;
     Document::setDesignMode(mode);
 }
 
 HTMLBodyElement* HTMLDocument::htmlBodyElement() const
 {
     HTMLElement* body = this->body();
-    return (body && body->hasTagName(bodyTag)) ? toHTMLBodyElement(body) : 0;
+    return isHTMLBodyElement(body) ? toHTMLBodyElement(body) : 0;
 }
 
 const AtomicString& HTMLDocument::bodyAttributeValue(const QualifiedName& name) const
 {
     if (HTMLBodyElement* body = htmlBodyElement())
         return body->fastGetAttribute(name);
     return nullAtom;
 }
 
 void HTMLDocument::setBodyAttribute(const QualifiedName& name, const AtomicString& value)
@@ skipping 184 matching lines @@
 void HTMLDocument::writeln(DOMWindow* callingWindow, const Vector<String>& text)
 {
     ASSERT(callingWindow);
     StringBuilder builder;
     for (size_t i = 0; i < text.size(); ++i)
         builder.append(text[i]);
     writeln(builder.toString(), callingWindow->document());
 }
 
 }