[giow] (3) Simplify task ordering in media elements to aid implementations.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=24710
Affected topics: Video and Audio

git-svn-id: http://svn.whatwg.org/webapps@8514 340c8d12-0b0e-0410-8428-c7bf67bfef74