ES6 will allow trailing commas in object literals.  It turns out that V8
already allowed it, too, as does JSC and SpiderMonkey.

R=marja@chromium.org
BUG=

Review URL: https://codereview.chromium.org/350353002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22005 ce2b1a6d-e550-0410-aec6-3dcde31c8c00