chromium/chrome/test/data/extensions/resource_timing/test-page.html

<!doctype html>
<html>

<head>
  <title>Resource Fetched That Are Initiated from Content Script</title>
</head>

<body>
  Such resource should not trigger performance timeline resource timing entry.
  <script>
    const getResourceTimingEntryCount = () => {
      return performance.getEntriesByType('resource').filter(e => e.name.includes('24.png')).length;
    }

    const getResourceTimingEntryCountAsync = async () => {
      return await new Promise(resolve => {
        new PerformanceObserver(entryList => {
          const length = entryList.getEntries().filter(e => e.name.includes('24.png')).length;
          if (length) {
            resolve(length);
          }
        }).observe({ type: 'resource', buffered: true });
      });
    }
  </script>
</body>

</html>