Improve String.repeat.

Adapted from patch contributed by Isiah Meadows <impinball@gmail.com>.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/657863002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24687 ce2b1a6d-e550-0410-aec6-3dcde31c8c00