Error stack getter should not overwrite itself with a data property.

R=ulan@chromium.org
BUG=v8:3294
LOG=Y

Review URL: https://codereview.chromium.org/258933007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21016 ce2b1a6d-e550-0410-aec6-3dcde31c8c00