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chromium
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chromium/third_party/blink/web_tests/wpt_internal/cookies/resources/iframe.html 

<!doctype html>
<meta charset="utf-8">
<script>
// Step 6 (wpt_internal/cookies/dt-coexist.{}.sub.https.window.js)
let visible_data = "";
if (document.cookie.includes("test")) {
    visible_data += "Cookie,";
}
if (window.localStorage.getItem("test") == "test") {
    visible_data += "LocalStorage,";
}

// Step 7 (wpt_internal/cookies/dt-coexist.{}.sub.https.window.js)
window.top.postMessage({type: "HadData", message:visible_data}, "*");
</script>