Add "own" symbols support.

"Own" symbols are symbols that can only denote own properties of
objects.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/464473002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23056 ce2b1a6d-e550-0410-aec6-3dcde31c8c00