This is a preliminary step before changing the way we store zone pointers in the zones.

Bug: v8:7903, v8:7754
Change-Id: I1e1af1823766c888ee0f8fe190f205f5b7e21973
Reviewed-on: https://chromium-review.googlesource.com/1118887Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54193}