Allocation site pretenuring for StringAdd on all platforms.

BUG=
R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/147763005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18882 ce2b1a6d-e550-0410-aec6-3dcde31c8c00