chrome/test/data/subresource_filter/page_with_websocket.html

<script src='websocket_connection.js'></script>
<script>
// Listens for 'onmessage' or 'onclose' messages from the websocket, and sends
// them to the test fixture.
function connectionListener(e) {
  if (e.data == 'onmessage') {
    window.domAutomationController.send(true);
  } else if (e.data == 'onclose') {
    window.domAutomationController.send(false);
  }
}

// This page has two modes, one where the websocket connection lives in the
// document context, and one where it lives in a worker. This is encoded by the
// presence (or absence) of an URL param named 'inWorker'.
var url = new URL(location);
var connectWebSocket;
if (url.searchParams.get('inWorker')) {
  var worker = new Worker('websocket_worker.js');
  worker.addEventListener('message', connectionListener);
  connectWebSocket = (url) => {
    worker.postMessage({url: url});
  };
} else {
  addEventListener('message', connectionListener);
  var messageCallback = (data) => { self.postMessage(data, location); }
  connectWebSocket = (url) => {
    connectWebSocketWithMessageCallback(url, messageCallback);
  };
}
</script>