The "places-shutdown" notification is not a good time to remove observers, since
history notifications (and in particular onClearHistory) may happen at the same
time (The sanitizer runs at "profile-change-teardown" that is basically the same
as "places-shutdown").
We can't ensure proper order of these calls, the safest/simplest thing is just
to keep a weak reference and continue observing.
MozReview-Commit-ID: 9zHkDqozXup
