add GetOwnPropertyDescriptor to api

R=verwaest@chromium.org

BUG=

Review URL: https://codereview.chromium.org/364923006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22190 ce2b1a6d-e550-0410-aec6-3dcde31c8c00