Harmony: implement Math.trunc.

BUG=v8:2938
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/28793002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17286 ce2b1a6d-e550-0410-aec6-3dcde31c8c00