LoadManagerSingleton has a separate shutdown path (xpcom-shutdown) from
its users (Audio/VideoConduit - garbage collected). These have appeared
racy, so in some cases the singleton was destructed before the users had
deregistered (e.g., when conduits destructed by SnowWhiteKiller).
A WeakPtr can solve this.
MozReview-Commit-ID: AVrpd3QqOGx
--HG--
extra : rebase_source : c5d50f7619940b772c7d1f3dee3ac0b6568ccfd9
