[giow] (2) Provide a way for scripts to know when TextTrackList members get enabled or disabled.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=22086
Affected topics: HTML, Video Text Tracks, Video and Audio

git-svn-id: http://svn.whatwg.org/webapps@7968 340c8d12-0b0e-0410-8428-c7bf67bfef74