Use unsigned integer arithmetic in Zone::NewExpand.
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: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@18564 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh