Introduce ES6 iterator for DOM objects.

This CL introduces blink::Iterator class and "Iterable" IDL attribute to make
it enable for DOM objects to become iteratable.

BUG=401699

Review URL: https://codereview.chromium.org/483163003

git-svn-id: svn://svn.chromium.org/blink/trunk@181294 bbb929c8-8fbe-4397-9dbb-9b2b20218538