We would like that to make sure every time we read map's prototype's map
we read the same one.

CL created after the discussion on
https://chromium-review.googlesource.com/c/v8/v8/+/2210233.

Bug: v8:7790
Change-Id: I4b6ffe733cf0b011b1bd1a3620ae8f1f35fa5c87
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2264101
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68555}