chromium/src/third_party/blink/web_tests/http/tests/devtools/application-panel/resources/indexeddb-in-iframe.html

<script>
  function makeDB() {
    return new Promise((resolve) => {
      indexedDB.open('Database-iframe').onupgradeneeded = event => {
          const db = event.target.result;
          const objectStore = db.createObjectStore('Database-iframe');
          objectStore.transaction.oncomplete = _ => {
            indexedDB.open('Database-iframe').onsuccess = event => event.target.result
              .transaction('Database-iframe', 'readwrite')
              .objectStore('Database-iframe')
              .add(940123, 'key');
          }
          resolve();
      }
    });
  }

  window.addEventListener("load", async function(e) {
    await makeDB();
  }, false);

</script>