Users of padded objects must know the actual object size for
implementing custom finalizers.

Bug: chromium:1056170
Change-Id: I0ddf9066cfece0a8d18a9e6fd985d09449eea92a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2644941
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72269}