[giow] (2) Make the third argument of pushState and replaceState accept null.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=17087
Affected topics: DOM APIs, HTML

git-svn-id: http://svn.whatwg.org/webapps@7685 340c8d12-0b0e-0410-8428-c7bf67bfef74