ExecutableAccessorInfo turns the property into a field. Better
to keep it as a callback, and correctly deal with the changed
property attributes.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/262053011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21558 ce2b1a6d-e550-0410-aec6-3dcde31c8c00