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chromium/third_party/blink/web_tests/external/wpt/cookies/samesite/resources/echo-cookies.html 

<!DOCTYPE html>
<meta charset="utf-8">
<script>
  if (window.opener)
    window.opener.postMessage({ type: 'COOKIES_SET', cookies: document.cookie }, '*');
  if (window.parent !== window)
    window.parent.postMessage({ type: 'FRAME_COOKIES_SET', cookies: document.cookie }, '*');
</script>