`DocumentObserver` register itself in Document` from constructor and
unregister itself from `finalize()`. The problem is that `finalize()`
won't be invoked for as long as `DocumentObserver` is visited by
`Document`. By not visiting registered observers from `Document` we
move this responsibility to object that allocated observer, which is
always exactly what we want, e.g. once `SVGUseElement` that uses
observer is gone, observer won't be visited anymore which will lead to
`finalize()` being called.
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