Ensure that we don't concurrently modify the StringsStorage map when
getting a copy of a string.

Bug: v8:11054
Change-Id: I6ad61838d7c5e8a6e9ff21aac04da8d353e41ad5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2648821Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Andrew Comminos <acomminos@fb.com>
Cr-Commit-Position: refs/heads/master@{#72416}