It added the __proto__ key as a normal key, which made it visible
in enumeration, while reading still hit the hard-coded accessor.

Review URL: http://codereview.chromium.org/6451002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6674 ce2b1a6d-e550-0410-aec6-3dcde31c8c00