[giow] (0) Be more specific about applying MEDIAFRAG rules to <video>.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23541
Affected topics: Video and Audio

git-svn-id: http://svn.whatwg.org/webapps@8254 340c8d12-0b0e-0410-8428-c7bf67bfef74