The zone_excess_limit_ is not changed at runtime, and there seems to
be no obvious reason to do so in the future.

R=danno@chromium.org
BUG=

Review URL: https://codereview.chromium.org/18034006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15354 ce2b1a6d-e550-0410-aec6-3dcde31c8c00