Fix ArrayLengthSetter to not throw on non-extensible receivers.

BUG=v8:3460
LOG=n
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/411983003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22576 ce2b1a6d-e550-0410-aec6-3dcde31c8c00