Fix stack trace accessor behavior.

R=verwaest@chromium.org
BUG=v8:3404
LOG=N

Review URL: https://codereview.chromium.org/343563009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22089 ce2b1a6d-e550-0410-aec6-3dcde31c8c00