This is a testing function used by d8 to test string externalization.

Bug: v8:12007
Change-Id: Ic19f28a42e1f9681ab08c00106788c569639fe7e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3888378
Commit-Queue: Adam Klein <adamk@chromium.org>
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83117}