MakeExternal can change byteness of strings without updating wrappers; hence we
cannot trust byteness of wrappers. Unwrap the string before checking encoding.

Bug: chromium:960680

Change-Id: I95d2bca906141c0d8b3de8fdac44b9fdb2ef2c43
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1601140Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61326}