chromium/src/third_party/blink/web_tests/http/tests/inspector-protocol/network/request-interception-mock404.js

(async function(testRunner) {
  var {page, session, dp} = await testRunner.startBlank(
      `Tests a mock 404 resource.`);

  var InterceptionHelper = await testRunner.loadScript('../resources/interception-test.js');
  var helper = new InterceptionHelper(testRunner, session);

  var requestInterceptedDict = {
    'redirect-iframe.html': event => helper.allowRequest(event),
    'redirect1.pl': event => helper.mockResponse(event, 'HTTP/1.1 404 Not Found\r\n\r\n'),
  };

  await helper.startInterceptionTest(requestInterceptedDict);
  session.evaluate(`
    var iframe = document.createElement('iframe');
    iframe.src = '${testRunner.url('./resources/redirect-iframe.html')}';
    document.body.appendChild(iframe);
  `);
})