Explicitly disallow stores to JSObjectAccessors (for now).
TBR=dcarney@chromium.org

Review URL: https://codereview.chromium.org/154873002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19086 ce2b1a6d-e550-0410-aec6-3dcde31c8c00