https://people.mozilla.org/~jorendorff/es6-draft.html#sec-set.prototype.keys

The value of the keys property is just the initial value of the values
function.

BUG=v8:3411
LOG=Y
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/353293003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22099 ce2b1a6d-e550-0410-aec6-3dcde31c8c00