Array.observe emit splices for array length change and update index >= length

R=adamk@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/15504002

Patch from Rafael Weinstein <rafaelw@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14944 ce2b1a6d-e550-0410-aec6-3dcde31c8c00