Section 15.14.5.10 and 15.16.5.7 in the October 26, 2012 ES6 draft,
http://wiki.ecmascript.org/doku.php?id=harmony:specification_drafts

This adds a getter for "size" to Set.prototype and Map.prototype which reflects
the number of elements in the Set and Map respectively.

BUG=v8:2395

Review URL: https://codereview.chromium.org/11360089
Patch from Erik Arvidsson <arv@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12875 ce2b1a6d-e550-0410-aec6-3dcde31c8c00