[ic] Minor constant cleanup.

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}

src/factory.cc

@@ -100,7 +100,7 @@ Handle<PrototypeInfo> Factory::NewPrototypeInfo() {
       Handle<PrototypeInfo>::cast(NewStruct(PROTOTYPE_INFO_TYPE, TENURED));
   result->set_prototype_users(WeakFixedArray::Empty());
   result->set_registry_slot(PrototypeInfo::UNREGISTERED);
-  result->set_validity_cell(Smi::kZero);
+  result->set_validity_cell(Smi::FromInt(Map::kPrototypeChainValid));
   result->set_bit_field(0);
   return result;
 }