Document ownership with using std::unique_ptr<Space> for the space_
array.

Bug: v8:13267
Change-Id: I12861d97cd52d2a8cf9ceb43a2f90008be87b2a3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3890913Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83187}