Use new is*Element() helper functions in CSS code

Source/core/css/StyleSheetList.cpp

 /**
  * (C) 1999-2003 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2006, 2007 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.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 58 matching lines @@
     if (!m_treeScope)
         return 0;
 
     // IE also supports retrieving a stylesheet by name, using the name/id of the <style> tag
     // (this is consistent with all the other collections)
     // ### Bad implementation because returns a single element (are IDs always unique?)
     // and doesn't look for name attribute.
     // But unicity of stylesheet ids is good practice anyway ;)
     // FIXME: We should figure out if we should change this or fix the spec.
     Element* element = m_treeScope->getElementById(name);
-    if (element && element->hasTagName(styleTag))
-        return toHTMLStyleElement(element);
-    return 0;
+    return isHTMLStyleElement(element) ? toHTMLStyleElement(element) : 0;
 }
 
 CSSStyleSheet* StyleSheetList::anonymousNamedGetter(const AtomicString& name)
 {
     HTMLStyleElement* item = getNamedItem(name);
     if (!item)
         return 0;
     return item->sheet();
 }
 
 void StyleSheetList::trace(Visitor* visitor)
 {
 }
 
 } // namespace WebCore