Just calculate pretenuring ratio when needed.

BUG=
R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/153863005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19144 ce2b1a6d-e550-0410-aec6-3dcde31c8c00