Expose the content of Sets and WeakSets through SetMirror.

BUG=v8:3093
LOG=Y
R=aandrey@chromium.org, arv@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/402423003

Patch from Alexandra Mikhaylova <amikhaylova@google.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22541 ce2b1a6d-e550-0410-aec6-3dcde31c8c00