Enforcing this invariant allows for assuming that free memory is left
untouched.

Bug: chromium:1056170
Change-Id: Ia225a31bbe6d394b8310ce512ed4f76f78e5c177
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3017808
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75669}