[giow] (2) Clear the 'audioTracks' and 'videoTracks' attributes when loading a new resource.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=19272
Affected topics: Video Text Tracks, Video and Audio

git-svn-id: http://svn.whatwg.org/webapps@7854 340c8d12-0b0e-0410-8428-c7bf67bfef74