Fixes Object::SetAccessor to return false if the prop is unconfigurable.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 9987d9b7163a9a169ed8d5adbff95e43d13b30c9..eb46da23c4459d4131d57c0815143303375f8855 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -4678,7 +4678,7 @@ static Maybe<bool> ObjectSetAccessor(Local<Context> context, Object* self,
   has_pending_exception =
       !i::JSObject::SetAccessor(obj, info).ToHandle(&result);
   RETURN_ON_FAILED_EXECUTION_PRIMITIVE(bool);
-  if (result->IsUndefined(obj->GetIsolate())) return Nothing<bool>();
+  if (result->IsUndefined(obj->GetIsolate())) return Just(false);
   if (fast) {
     i::JSObject::MigrateSlowToFast(obj, 0, "APISetAccessor");
   }