[giow] (2) Try to define what happens when you have tracks that don't cover the whole media timeline
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=26460
Affected topics: Video and Audio

git-svn-id: http://svn.whatwg.org/webapps@8754 340c8d12-0b0e-0410-8428-c7bf67bfef74