static requestAnimationTicker() {
        return new Observable(observer => {
            let iter;
            let isDone = false;
            iter = () => {
                if (!isDone) {
                    observer(undefined);
                    window.requestAnimationFrame(iter);
                }
            };
            iter();
            return () => { isDone = true; };
        });
    }
Returns an observable that keeps requesting animations frame callbacks and calling observers when they arrive.
@returns {!Observable.<undefined>}