This is a reland of commit 6d342fa5

Original change's description:
> [heap] Use std::unique_ptr for space_ array
>
> 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/+/3890913
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#83187}

Bug: v8:13267
Change-Id: Idb25a656c4ba571d23132aa5e07cb13957c90f0b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899121Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83259}