Properly scope unique_ptr for Heap.

Change-Id: I9ce65f326065333f2600e6057ae3015a41d4c39a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3273815
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77839}