postMessage doesn't really have different origins for file: URIs;
to send to them you need "*", and `location.origin` is null. So
testing jar:file separately doesn't seem to make much sense, so
let's just remove the test now that jar: will only really be
usable with a local/file inner URI.
MozReview-Commit-ID: 7vvlrugi3xv
--HG--
extra : rebase_source : cfa701fd5fa03bf8d03a2bd773ec908a26020944
