[giow] (2) Add initMessageEvent()
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=25907
Affected topics: DOM APIs

git-svn-id: http://svn.whatwg.org/webapps@8711 340c8d12-0b0e-0410-8428-c7bf67bfef74
Merge remote branch 'origin/master' into whatwg