This adds a flag behind which we want to do the work towards allowing
prototype objects to stay in dict/"slow" mode rather than switching
them back to fast mode

Bug: v8:7569

Change-Id: I3c963dea5d01be3c348810f40f8610fc2a488819
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2450015Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Frank Emrich <emrich@google.com>
Cr-Commit-Position: refs/heads/master@{#70367}