Use C++11 features in core/html

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 248 matching lines @@
         return parent->fastGetAttribute(HTMLNames::crossoriginAttr);
 
     return nullAtom;
 }
 
 HTMLMediaElement* HTMLTrackElement::mediaElement() const
 {
     Element* parent = parentElement();
     if (isHTMLMediaElement(parent))
         return toHTMLMediaElement(parent);
-    return 0;
+    return nullptr;
 }
 
 void HTMLTrackElement::trace(Visitor* visitor)
 {
     visitor->trace(m_track);
     HTMLElement::trace(visitor);
 }
 
 }