createTracker = page => {
  const requests = new Set();
  const onStarted = request => requests.add(request);
  const onFinished = request => requests.delete(request);
  page.on("request", onStarted);
  page.on("requestfinished", onFinished);
  page.on("requestfailed", onFinished);
  return {
    urls: () => Array.from(requests).map(r => r.url()),
    dispose: () => {
      page.removeListener("request", onStarted);
      page.removeListener("requestfinished", onFinished);
      page.removeListener("requestfailed", onFinished);
    }
  };
}
Sets up event listeners on the Browser.Page instance to maintain a set
of URLs that have started but never finished or failed.

@param {Object} page
@return Object