ES6: Implement WeakMap and WeakSet constructor logic

Now that iterators are enabled by default we need to correctly
handle the parameter for WeakMap and WeakSet. If provided then the
argument is iterated over to add entries to the WeakMap and WeakSet.

BUG=v8:3399
LOG=Y
R=adamk@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/448013005

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22999 ce2b1a6d-e550-0410-aec6-3dcde31c8c00