Since we have synchronized versions of map and length we can use those.
For reading the contents, however, we have to add a lock around
externalizing a string.

Bug: v8:7790
Change-Id: Iedcb6d9b865d80fbe6d8aec5dd677943ab9ac1d0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2497179Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71010}