Replace Element::isMediaElement() with isHTMLMediaElement

Source/core/html/HTMLTrackElement.cpp

 /*
  * Copyright (C) 2011 Google 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 249 matching lines @@
 {
     if (HTMLMediaElement* parent = mediaElement())
         return parent->fastGetAttribute(HTMLNames::crossoriginAttr);
 
     return nullAtom;
 }
 
 HTMLMediaElement* HTMLTrackElement::mediaElement() const
 {
     Element* parent = parentElement();
-    if (parent && parent->isMediaElement())
-        return toHTMLMediaElement(parentNode());
+    if (isHTMLMediaElement(parent))
+        return toHTMLMediaElement(parent);
     return 0;
 }
 
 }