ES6: Add support for values/keys/entries for Map and Set

This allows code like this:

  var map = new Map();
  map.set(1, 'One');
  ...
  var iter = map.values();
  var res;
  while (!(res = iter.next()).done) {
    print(res.value);
  }

BUG=v8:1793
LOG=Y
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/259883002

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21615 ce2b1a6d-e550-0410-aec6-3dcde31c8c00