When a debug event is triggered, the ExecutionState object should not
expose native JS code.

R=aandrey@chromium.org

Review URL: https://codereview.chromium.org/429453005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22849 ce2b1a6d-e550-0410-aec6-3dcde31c8c00