Fix issue 186:

Fix issue 186:

    http://code.google.com/p/v8/issues/detail?id=186

Create a new instance type for context extension objects.  Use it to
not use the __proto__ accessor for context extension objects.
Review URL: http://codereview.chromium.org/18044

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1072 ce2b1a6d-e550-0410-aec6-3dcde31c8c00