Before this change, we would enumerate all the keys with
[[OwnPropertyKeys]], and then do [[GetOwnPropertyDescriptor]] twice for
each key as we went through them.
We now only do one [[GetOwnPropertyDescriptor]] per key, which
drastically reduces the number of proxy traps when those are involved.
The new trap sequence matches what you get with V8, so I don't think
anyone will be unpleasantly surprised here.
