When a background page has a messaging listener but the background page
is started in response to something other than an extension message, we
were missing the background "startup" event. Fix that by setting up the
listener earlier.
MozReview-Commit-ID: Cr58EyCoY6W
--HG--
extra : rebase_source : 0d309e01be35e2ef7ab5e489726d0191571ca371