WeakSets work similar to ordinary Sets but the value (which must be an
object) is held weakly.

This is available under --harmony-collections

BUG=v8:2785
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/19678023

Patch from Erik Arvidsson <arv@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15792 ce2b1a6d-e550-0410-aec6-3dcde31c8c00