ResizeObserver = function(callback) {
    if (!(this instanceof ResizeObserver)) {
        throw new TypeError('Cannot call a class as a function.');
    }
    if (!arguments.length) {
        throw new TypeError('1 argument required, but only 0 present.');
    }

    var controller = ResizeObserverController.getInstance();
    var observer = new ResizeObserverSPI(callback, controller, this);

    observers.set(this, observer);
}
ResizeObserver API. Encapsulates the ResizeObserver SPI implementation
exposing only those methods and properties that are defined in the spec.