Make ReturnValue a friend of Persistent, so it can use operator*.

BUG=
R=dcarney@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/15848010

Patch from Marja Hölttä <marja@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14999 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

include/v8.h

@@ -796,6 +796,7 @@ template <class T> class Persistent // NOLINT
  private:
   template<class F> friend class Handle;
   template<class F> friend class Local;
+  template<class F> friend class ReturnValue;
   friend class ImplementationUtilities;
   friend class ObjectTemplate;
   friend class Context;