Fix 3.20 performance regression

Needing to Local<T>::New() was causing a performance regression. From a
discussion on the v8-users mailing list I was told the Chrome team is
using a similar technique to get around the issue.

Mailing List:
  https://groups.google.com/d/msg/v8-users/6kSAbnUb-rQ/9G5RmCpsDIMJ