Increase precision for base conversion for large integers.

R=jkummerow@chromium.org
BUG=v8:3025
LOG=Y

Review URL: https://codereview.chromium.org/88583002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18082 ce2b1a6d-e550-0410-aec6-3dcde31c8c00