Concurrently accessing internal external uncached strings is not
thread-safe. We are removing a case where we can make such a string
through MakeExternal.

Bug: v8:7790
Change-Id: I958062c15cf40ccc330600bb572de98620866e54
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2565511
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71573}