Extend the interceptor setter ASSERT to support the JSGlobalProxy case.

BUG=v8:3463
LOG=n
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/415973004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22589 ce2b1a6d-e550-0410-aec6-3dcde31c8c00