By matching ZoneHandleSet's internals to Handle's internals,
we can avoid a few unnecessary casts, and get rid of a few
more occurrences of Object**.

Bug: v8:3770
Change-Id: I24d6bad3a4959d977abf1f6b8db9be50bb2bc6d8
Reviewed-on: https://chromium-review.googlesource.com/c/1330907
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57418}