That's the only place where it's used, with a known StringTableKey type.
This also makes MakeThin available on String.

Change-Id: If1aca892045dadae91c902e783822280ae01f485
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1598764
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61299}