Object.defineProperty with empty property descriptor.

The issue is fixed by implementing step 5 and 6 from DefineOwnProperty in the
specification (ES5 8.12.9).

This also fixes a bug in SameValue when used on boolean values (it
would priorly return a number - not a boolean).

Review URL: http://codereview.chromium.org/2131019

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4708 ce2b1a6d-e550-0410-aec6-3dcde31c8c00