[giow] (3) appcache: coalesce queued progress events.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=17688
Affected topics: Offline Web Applications

git-svn-id: http://svn.whatwg.org/webapps@8751 340c8d12-0b0e-0410-8428-c7bf67bfef74