[giow] (3) Make appcache requests have no referrer, since it's not clear what referrer to use.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=17687
Affected topics: HTML, Offline Web Applications

git-svn-id: https://svn.whatwg.org/webapps@8791 340c8d12-0b0e-0410-8428-c7bf67bfef74