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 <div> is useless, but harmless.
Merge pull request #105 from foolip/rm-webkit-enter-fullscreen-div

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