Bug 1057966 - Fix Help menu in e10s windows. r=billm.

The help menu wasn't populating properly because gSafeBrowsing was attempting
to get the URI for the loaded page by getting content.document.documentURI.
Since bug 1051017 landed, content is unavailable for e10s windows. We use
the selected browser's currentURI instead.

--HG--
extra : rebase_source : 7dfa0f74b99ed35563bc4507c2a5ff70a703af3e