Add missing lock in FakeObserver
Add missing lock in FakeObserver

It looks a lot like a missing lock, at least.

I got many "unrecognized selector" failures on the line that accesses
the watchedElements dictionary in
mainWindow__when_a_new_window_becomes_main_then_closes_before_reading_the_attribute__updates_the_value_correctly,
and saw one segfault in
mainWindow__when_a_new_window_becomes_main_then_closes_after_reading_the_attribute__updates_the_value_correctly.

tmandry/Swindler,tmandry/Swindler,tmandry/Swindler