Add infrastructure to keep track of references to prototypes.

src/runtime/runtime-debug.cc

Index: src/runtime/runtime-debug.cc
diff --git a/src/runtime/runtime-debug.cc b/src/runtime/runtime-debug.cc
index 65ad4452c2fc76f9ae31b9b83499de9d875feea7..12c5a0d84f3567a2690bdd0fb1e6bee107f6e81d 100644
--- a/src/runtime/runtime-debug.cc
+++ b/src/runtime/runtime-debug.cc
@@ -2169,7 +2169,7 @@ static Handle<JSObject> NewJSObjectWithNullProto(Isolate* isolate) {
       isolate->factory()->NewJSObject(isolate->object_function());
   Handle<Map> new_map =
       Map::Copy(Handle<Map>(result->map()), "ObjectWithNullProto");
-  new_map->set_prototype(*isolate->factory()->null_value());
+  new_map->SetPrototype(isolate->factory()->null_value());
   JSObject::MigrateToMap(result, new_map);
   return result;
 }