This patch causes V8 to disable EXTRA_CHECKS when building in release. You can
still enable the checks in release using a GYP flag.

This patch speeds up Dromeo's dom-traverse by around 4%.

Review URL: https://codereview.chromium.org/11275324
Patch from Adam Barth <abarth@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12981 ce2b1a6d-e550-0410-aec6-3dcde31c8c00