sequentialFetch = (iframe, urls) => {
    return urls.reduce((chain, url) => {
      return chain.then(() => iframe.contentWindow.fetch(url));
    }, Promise.resolve()).then(() => pause(500));
  }
Performs a series of fetch() calls on an iframe, then pauses.
@param {iframe} iframe The iframe whose contentWindow will be used to fetch().
@param {Array.<String>} urls The URLs to fetch.
@return {Promise} A promise that resolves following the fetches and a delay.