Ensure the receiver for EnableAccessCheck is always a JSObject

BUG=
TBR=dcarney@chromium.org

Review URL: https://codereview.chromium.org/349923005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21929 ce2b1a6d-e550-0410-aec6-3dcde31c8c00