[giow] (3) Make .crossOrigin = null remove the content attribute.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=26105
Affected topics: DOM APIs, HTML, Video and Audio

git-svn-id: http://svn.whatwg.org/webapps@8727 340c8d12-0b0e-0410-8428-c7bf67bfef74