Move Maps' back pointers from "transitions" to "constructor" field

src/runtime/runtime-scopes.cc

Index: src/runtime/runtime-scopes.cc
diff --git a/src/runtime/runtime-scopes.cc b/src/runtime/runtime-scopes.cc
index cffd7e1e0185c9e1f4ee2f06c39ef57f1dd166d1..201bf9d4c683030fcdcc3a15d94a7b51b02869ca 100644
--- a/src/runtime/runtime-scopes.cc
+++ b/src/runtime/runtime-scopes.cc
@@ -863,7 +863,7 @@ static Object* ComputeReceiverForNonGlobal(Isolate* isolate, JSObject* holder) {
   // as the receiver. This allows arguments objects to be used as
   // receivers, but only if they are put in the context scope chain
   // explicitly via a with-statement.
-  Object* constructor = holder->map()->constructor();
+  Object* constructor = holder->map()->GetConstructor();
   if (constructor != context_extension_function) return holder;
   // Fall back to using the global object as the implicit receiver if
   // the property turns out to be a local variable allocated in a