code: function addDelegatedListener(el, type, listener) {
    incrementListenerCount(listener);
    return el.addEventListener(type, getDelegateFunction_(listener));
  }
docs: Adds an event listener to an element where the event target
  is discovered by moving up the clicked element's descendants.
  @param {Element} el Element to host the listener.
  @param {string} type Listener type.
  @param {Function} listener Listener function.