The service worker job queue can get stuck sometimes, so we don't want
the per-process service worker data structure to hold strong
references to content. Instead, I add a proxy that is only a weak
reference. The wrinkle is that we need to keep the promise alive as
long as the job and the window are otherwise alive. I solve this by
putting a cycle collected reference to the promise on the window
itself.
MozReview-Commit-ID: GbmCY4ZIQWk
--HG--
extra : rebase_source : 363ea865fd51bb44f3e727cd3469e316370e7cbc
