broadcastActive() {
        // Do nothing if observer doesn't have active observations.
        if (!this.hasActive()) {
            return;
        }

        const ctx = this.callbackCtx_;

        // Create ResizeObserverEntry instance for every active observation.
        const entries = this.activeObservations_.map(observation => {
            return new ResizeObserverEntry(
                observation.target,
                observation.broadcastRect()
            );
        });

        this.callback_.call(ctx, entries, ctx);
        this.clearActive();
    }
Invokes initial callback function with a list of ResizeObserverEntry
instances collected from active resize observations.

@returns {void}