Walk up frame tree for srcdoc referrer policies

third_party/WebKit/LayoutTests/http/tests/security/resources/referrer-policy-srcdoc.php

+window.addEventListener("message", function (event) {
+  if (event.data.command == "send-fetch-referrer") {
+    fetch(event.data.url).then(function (response) {
+      response.text().then(function (responseText) {
+        window.top.postMessage({
+          test: "send-fetch-referrer",
+          referrer: responseText
+        }, "*");
+      });
+    });
+  }
+});
+
+window.top.postMessage({
+  test: '<?php echo $_GET["test"] ?>',
+  referrer: '<?php echo $_SERVER["HTTP_REFERER"] ?>'
+}, "*");