Callbacks that run after garbage collection might fire a context dispose
notification and hence increse the counter before we actually reach this
position.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/11280214

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13080 ce2b1a6d-e550-0410-aec6-3dcde31c8c00