Further ElementsAccessor handlification (GetKeyForIndex(), GetCapacity(), GetType() and GetAttributes()).

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/228643003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20606 ce2b1a6d-e550-0410-aec6-3dcde31c8c00