Tweaks to the code serializer.

- consider the source string as a special sort of back reference.
- use repeat op code for more root members.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/674883002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24871 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Enable --serialize-toplevel by default.

R=vogelheim@chromium.org

Review URL: https://codereview.chromium.org/661343003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24872 ce2b1a6d-e550-0410-aec6-3dcde31c8c00