ResizeObserverController = function() {
    this.connected_ = false;
    this.mutationEventsAdded_ = false;
    this.mutationsObserver_ = null;
    this.observers_ = [];

    this.onTransitionEnd_ = this.onTransitionEnd_.bind(this);
    this.refresh = throttle(this.refresh.bind(this), REFRESH_DELAY);
}
Singleton controller class which handles updates of ResizeObserver instances.