There is already a %GetUndetectable runtime test function, so use that.

Bug: v8:7184
Change-Id: I04af03d95c4245ab9c7061cb00d5890972b82f46
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2181195
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67553}