Array.prototype.{slice,splice} should use [[DefineOwnProperty]] to generate return value

BUG=chromium:423633
LOG=N
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/649063003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24784 ce2b1a6d-e550-0410-aec6-3dcde31c8c00