Change Object.defineProperty to accept undefined as getters and setters and to correctly accept overriding an accessor with a data property.

In the past we only accepted functions as argument for setting an
accessor. Since one should be able to set an accessor to undefined
this had to be changed to take either.

In addition, we did not lookup properties in the prototype chain,
causing us to call the setter of an existing accessor up the prototype
chain when trying to replace an existing accessor (that was not local)
with a data property.


Review URL: http://codereview.chromium.org/5861006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6045 ce2b1a6d-e550-0410-aec6-3dcde31c8c00