[giow] (3) Change 'description' to the plural form so that videoTrack.kind is consistent with textTrack.kind, as it is for captions and subtitles already
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23239
Affected topics: Video and Audio

git-svn-id: http://svn.whatwg.org/webapps@8195 340c8d12-0b0e-0410-8428-c7bf67bfef74