'autoplay' starts when the ready state reaches HAVE_ENOUGH_DATA but the OSX media engine doesn't set readyState to HAVE_ENOUGH_DATA until the entire file has been loaded.
<rdar://problem/6697754>
Created attachment 29246 [details] proposed patch
Comment on attachment 29246 [details] proposed patch r=me, but it would be nice if we could have LayoutTests to test slower loading movies.
http://trac.webkit.org/changeset/42320