Use unsigned integer arithmetic in Zone::NewExpand.

BUG=328202
R=jkummerow@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/108783005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18564 ce2b1a6d-e550-0410-aec6-3dcde31c8c00