addObserver(observer) {
        if (!~this.observers_.indexOf(observer)) {
            this.observers_.push(observer);
        }

        // Add listeners if they haven't been added yet.
        if (!this.connected_) {
            this.connect_();
        }
    }
Adds observer to observers list.

@param {ResizeObserverSPI} observer - Observer to be added.
@returns {void}