Stack traces exposed to Javascript should omit extensions.

R=marja@chromium.org
BUG=v8:311
LOG=Y

Review URL: https://codereview.chromium.org/363893003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22162 ce2b1a6d-e550-0410-aec6-3dcde31c8c00