The tests were already passing because they happened to use objects
allocated in the lower half of the heap reservation, but this small
change should make behavior more consistent.

Change-Id: Ib6be3123d347234f4771c213f2209bfe6e19c569
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1860332Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#64294}