[giow] (0) Reword the track processing model to be sane.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=24487
Affected topics: Video Text Tracks

git-svn-id: http://svn.whatwg.org/webapps@8461 340c8d12-0b0e-0410-8428-c7bf67bfef74