... but use proper map for functions with readonly prototype from the start.

Bug: v8:6459
Change-Id: I432d4969822e7cc4c2ba83e103f550d1c4f2e234
Reviewed-on: https://chromium-review.googlesource.com/563199Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46487}