of intptr_t

This prevents an overflow in FreeArrayBuffer, which in turn caused needless GCs
as well as crashes on isolate teardown.

LOG=Y
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/70233010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17944 ce2b1a6d-e550-0410-aec6-3dcde31c8c00