Enable ES6 iteration by default

This enables for-of, as well as @@iterator implementations for strings
and arrays.

R=rossberg@chromium.org
BUG=v8:2214
LOG=Y

Review URL: https://codereview.chromium.org/446023002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22980 ce2b1a6d-e550-0410-aec6-3dcde31c8c00