It acquires the string lock to avoid race conditions. It does so in a
slow way (by getting the isolate from the string) to avoid piping the
Isolate through.

Bug: v8:7790, chromium:1166095
Change-Id: I8b769b4e96ee780314359d1d15d712012aade88a
Fix: chromium:1166095
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2637861Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72236}