We are guaranteed to have a valid ref for the prototype now that the
no-concurrent-inlining configuration has been removed.

Bug: v8:7790
Change-Id: I8400d1887f5cd41b14c92c87151847c0ed78f911
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3394708Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78648}