We can keep the non-atomic accessors for read/write since we set the
prototype on the map at initialization.

Bug: v8:7790, chromium:1150811
Change-Id: Ied7763c87a71c6aa93099dec3405873ab7419643
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2773052
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73615}