when setting a __proto__ to an object that holds a setter.

This seems to cause a major page load regression, so we need to tune
the clearing.
Review URL: http://codereview.chromium.org/50011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1548 ce2b1a6d-e550-0410-aec6-3dcde31c8c00