[giow] (3) Make content attribute event handlers work slightly more like in more browsers.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23956
Affected topics: DOM APIs

git-svn-id: http://svn.whatwg.org/webapps@8376 340c8d12-0b0e-0410-8428-c7bf67bfef74