Contexts for active extension views are kept in a Set on the owning extension.
That list is meant to be kept current, with views added and removed as they're
created and unloaded. A refactoring at some point in the past, though, changed
that so that we only cleaned up parent views at extension shutdown, not at
view shutdown.
MozReview-Commit-ID: FW8KHPOD9qc
--HG--
extra : rebase_source : fab255ba2fb5ee55be41c252c89930d38f6edbe8
extra : amend_source : 54863d79a1d571a7354aa15f74e2fc4448297777
27b1c032a5