As the proposal says:
Despite the obvious temptation to incorporate existing code into the new project, the intention is to start a new code base. Since Rhino things have moved on in Java's ability to support dynamic languages and API calls such as InvokeDynamic should result in faster and more reliable code without the memory leaks that plagued Rhino.
Introduction to Nashorn
Oracle already has some code that it plans to share if the project is voted into existence. The vote is for current OpenJDK members and closed on December 6th.