REGRESSION (r148073): WebKit Nightly r148082 crashes on launch in JSObjectSetPrivate
https://bugs.webkit.org/show_bug.cgi?id=114341

Reviewed by Alexey Proskuryakov.

Make JSObjectSetPrivate use uncheckedToJS as some clients
clear their private data during finalization for some reason.

* API/JSObjectRef.cpp:
(JSObjectSetPrivate):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148106 268f45cc-cd09-0410-ab3c-d52691b4dbfc