Also properly implement MapRef::GetInObjectPropertyOffset.

Bug: v8:7790
Change-Id: I3e73247df67b7c40336b6c685b784dbf9e542340
Reviewed-on: https://chromium-review.googlesource.com/1226977
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55948}