Allow Object.defineProperty to update value of an API accessor.

This is needed for converting internal accessors to API accessors and can break blink tests.

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/240573004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20910 ce2b1a6d-e550-0410-aec6-3dcde31c8c00