Don't look for webkitEnterFullscreen/webkitEnterFullScreen on <div>

These two functions have never been exposed on HTMLDivElement, only
on HTMLMediaElement/HTMLVideoElement:
http://trac.webkit.org/changeset/50893
http://trac.webkit.org/changeset/54143

Looking for it on the _playerElement &lt;div> is useless, but harmless.