There's no need for it to be a JSArray. In the spec it's a List
which roughly corresponds to FixedArray (but not JSArray).

Gotchas:
- There's no good point in time where we know the array length, since
resolving might be interleaved with iteration.
- Using ExtractFixedArray in places where we don't need to extract,
since it takes care of things like allocating the resulting array
in the right space etc.

Drive-by fix: Previously we pre-allocated an array 1 elements too big,
but never noticed it since the last element was the hole.

Bug: v8:10506
Change-Id: I6a72fbf1fc0cc031f2c8bad9314c4ed21d544a0f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2202905Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67919}