Use WeakCell to handle the script wrapper cache

The script wrapper cache used the API weak handles to provide a weak link from Script to ScriptWrapper. We want to change the way API weakness works, and in this context it's best to get rid of users of the API that don't need to be users.

R=ulan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/659513003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24622 ce2b1a6d-e550-0410-aec6-3dcde31c8c00