static getURLsAttemptedToPreload(networkRecords) {
    const attemptedURLs = networkRecords
      .filter(req => req.resourceType === 'Font')
      .filter(req => req.isLinkPreload)
      .map(req => req.url);

    return new Set(attemptedURLs);
  }
Finds which font URLs were attempted to be preloaded,
ignoring those that failed to be reused and were requested again.
Note: document.fonts.load() is a valid way to preload fonts,
but we are not currently checking for that.
@param {Array<LH.Artifacts.NetworkRequest>} networkRecords
@return {Set<string>}