Serialize code stubs using stub key.

Also add some tracing to the code serializer.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/556243005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24002 ce2b1a6d-e550-0410-aec6-3dcde31c8c00