Wait until the test page is fully loaded before dumping the contents.

third_party/WebKit/LayoutTests/http/tests/security/referrer-policy-attribute-img-no-referrer-when-downgrade.html

Index: third_party/WebKit/LayoutTests/http/tests/security/referrer-policy-attribute-img-no-referrer-when-downgrade.html
diff --git a/third_party/WebKit/LayoutTests/http/tests/security/referrer-policy-attribute-img-no-referrer-when-downgrade.html b/third_party/WebKit/LayoutTests/http/tests/security/referrer-policy-attribute-img-no-referrer-when-downgrade.html
index a461f7242eb8159b944b37116e4e22013eefe9e4..a3d0297ee783aa527cff97fdf2aa9756f4d27675 100644
--- a/third_party/WebKit/LayoutTests/http/tests/security/referrer-policy-attribute-img-no-referrer-when-downgrade.html
+++ b/third_party/WebKit/LayoutTests/http/tests/security/referrer-policy-attribute-img-no-referrer-when-downgrade.html
@@ -12,11 +12,17 @@
             window.location = get_host_info().HTTPS_ORIGIN + window.location.pathname;
         } else {
             var img = document.createElement("img");
+
+            // Before calling testRunner.notifyDone (and taking a screenshot of
+            // the page), we need to make sure the image got actually loaded.
+            img.addEventListener("load", function() {
+                testRunner.notifyDone();
+            });
+
             img.src = "http://127.0.0.1:8000/security/resources/green-if-no-referrer.php";
             img.referrerpolicy = "no-referrer-when-downgrade";
             document.body.appendChild(img);
-            testRunner.notifyDone();
         }
     </script>
 </body>
-</html
+</html>