Make sure that the overflow store buffer size is always larger or equals than the regular store buffer size.

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/372983005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22278 ce2b1a6d-e550-0410-aec6-3dcde31c8c00