There's no reason for it to be a JSArray; it's an internal data member.

We only have a slow version for constructing a FixedArray from an
Iterable, but since this is not a performance critical code path,
it's fine.

BUG=v8:9808

Change-Id: I5bcfac4cc545880966421a96d3aa3d899bbf6cd5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2157371Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67321}