chromium/chrome/test/data/extensions/platform_apps/web_view/shim/echo.html

<!doctype html>
<!--
Copyright 2020 The Chromium Authors
Use of this source code is governed by a BSD-style license that can be
found in the LICENSE file.
-->
<html>
<head>
  <title>A guest that echoes postMessages.</title>
  <script>
    window.addEventListener('message', (e) => {
      e.source.postMessage(e.data, e.origin);
    });
  </script>
</head>
</html>