This is a preliminary step before we stop swapping maps in the bootstrapper
(strict/sloppy map with writable prototype <-> readonly prototype).

Bug: v8:6459
Change-Id: I120550c10e98a234e283d79a8d408096601c92af
Reviewed-on: https://chromium-review.googlesource.com/558879Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46403}