Make Map.set() and Set.add() chainable

From the Harmony draft:

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-map.prototype.set
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-weakmap.prototype.set
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-set.prototype.add
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-weakset.prototype.add

BUG=v8:3410
R=arv@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/359173002

Patch from caitp <caitpotter88@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22111 ce2b1a6d-e550-0410-aec6-3dcde31c8c00