Valid prototype chain validity cells should have an initial value of
"Map::kPrototypeChainValid", not zero (even though they're the same).

Bug: 
Change-Id: I7d3df7d2e3382f20ed598b387612bb48428e0fa0
Reviewed-on: https://chromium-review.googlesource.com/757140Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49196}