Reading the descriptor array from a map has been safe for a while.

Bug: v8:7790
Change-Id: Ib06e12727b7da26c09822db45530addc11e2cf00
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2739637Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73296}