Use the "convert" parameter to SequentialStringKey to construct one-byte
strings out of two-byte input vectors, where appropriate.

Change-Id: I8a214b3960c677614d6f82ed3b29405e2e493e81
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2557981
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81062}