This is also pre-work for in-place weak refs.

BUG=v8:7308, v8:5988

Change-Id: Ie78b0c59695c1e6af9780fffb363c931e2ee34e2
Reviewed-on: https://chromium-review.googlesource.com/1013583
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52658}