[giow] (2) Make <video> and <audio> sniff for their media data's MIME type, since most browsers seem to be going that way, and we need to pick a direction.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=11984
Affected topics: HTML, Video and Audio

git-svn-id: http://svn.whatwg.org/webapps@8696 340c8d12-0b0e-0410-8428-c7bf67bfef74
Merge remote branch 'origin/master' into whatwg