ES6 symbols: filter symbols form for-in loops and Object.keys

R=verwaest@chromium.org
BUG=v8:2158

Review URL: https://codereview.chromium.org/12455002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13838 ce2b1a6d-e550-0410-aec6-3dcde31c8c00