Previously such maps were marked as prototype, but that has bad
performance / memory characteristics if objects are used as
dictionaries.

Bug: b:148346655, v8:10339
Change-Id: I287c5664c8b7799a084669aaaffe3affcf73e95f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2179322Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67537}