Functions that don't have prototype need to store neither prototype nor
initial map, so the |prototype_or_initial_map| field is not required for
such maps.

Bug: v8:6459
Change-Id: I4b3066bd6a4fed42c19f217bae82a8bce552bdca
Reviewed-on: https://chromium-review.googlesource.com/570250Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46840}