Fix containing() w/ n point to surrogate tail.

Bug: v8:6891
Change-Id: I4d7e4083ba409eecaefaf2ec5c67b7226bd7c97c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2348208
Commit-Queue: Frank Tang <ftang@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69363}