This speeds up fast-mode object copying by ~2x and __proto__:null cases by ~20x.

Bug: 
Change-Id: Ic3893ae4c87063f0c8c524ef55c0e786b7a87192
Reviewed-on: https://chromium-review.googlesource.com/516983
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45536}