Pre-work for in-place weak refs.

BUG=v8:7308

Change-Id: I5c7086bded14879f62ab366d0aba59302f0078d3
Reviewed-on: https://chromium-review.googlesource.com/1010069
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52681}