Don't treat new prototypes differently depending on how they become a
prototype. This is work towards always keeping prototypes in slow-mode.


Bug: v8:6471
Change-Id: I62de1018e21d91fda3a5da044615f32c718910b1
Reviewed-on: https://chromium-review.googlesource.com/526596Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45781}