[e] (0) Fix IDL for 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@8760 340c8d12-0b0e-0410-8428-c7bf67bfef74