code: function(selector, className, opt_offset, opt_delay, opt_fromTop) {
    this.selector = selector;
    this.className = className;
    this.offset = opt_offset || null;
    this.delay = opt_delay || 0;
    this.fromTop = opt_fromTop || false;
  }
docs: Adds a class to elements when they become visible in the viewport.
  @param {string} selector Query selector to find elements to act upon.
  @param {string} className Class name to add to matched elements.
  @param {number=} opt_offset Offset (by percentage of the element) to
  apply when checking if the element is in view.
  @param {Array.<number>=} opt_delay An array of [min delay, max delay].
  @param {Boolean} opt_fromTop Whether to check if only the tops and bottoms
  of the elements are in view, and to skip checking the sides.