How can I add predefined length to audio recorded from MediaRecorder in Chrome?

var recorder,
  chunks = [],
  ctx = new AudioContext(),
  aud = document.getElementById('aud');

function exportAudio() {
  var blob = new Blob(chunks);
  aud.src = URL.createObjectURL(new Blob(chunks));

  aud.onloadedmetadata = function() {
    // it should already be available here
    log.textContent=" duration: " + aud.duration;
    // handle chrome's bug
    if (aud.duration === Infinity) {
      // set it to bigger than the actual duration
      aud.currentTime = 1e101;
      aud.ontimeupdate = function() {
        this.ontimeupdate = () => {
          return;
        }
        log.textContent += ' after workaround: ' + aud.duration;
        aud.currentTime = 0;
      }
    }
  }
}

function getData() {
  var request = new XMLHttpRequest();
  request.open('GET', 'https://upload.wikimedia.org/wikipedia/commons/4/4b/011229beowulf_grendel.ogg', true);
  request.responseType="arraybuffer";
  request.onload = decodeAudio;
  request.send();
}


function decodeAudio(evt) {
  var audioData = this.response;
  ctx.decodeAudioData(audioData, startRecording);
}

function startRecording(buffer) {

  var source = ctx.createBufferSource();
  source.buffer = buffer;
  var dest = ctx.createMediaStreamDestination();
  source.connect(dest);

  recorder = new MediaRecorder(dest.stream);
  recorder.ondataavailable = saveChunks;
  recorder.onstop = exportAudio;
  source.start(0);
  recorder.start();
  log.innerHTML = 'recording...'
  // record only 5 seconds
  setTimeout(function() {
    recorder.stop();
  }, 5000);
}

function saveChunks(evt) {
  if (evt.data.size > 0) {
    chunks.push(evt.data);
  }

}

// we need user-activation
document.getElementById('button').onclick = function(evt){
  getData();
  this.remove();
}

<button id="button">start</button>
<audio id="aud" controls></audio><span id="log"></span>