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..5a56fbe793197c9455751a878d2773adf644b3a3 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
@@ -15,8 +15,14 @@
             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();
+
+            // Before calling testRunner.notifyDone (and taking a screenshot of
+            // actual.png), we need to make sure the image appended above got a
+            // chance to paint itself.
+            testRunner.layoutAndPaintAsyncThen(function() {

[dcheng, 2016/04/25 23:15:03]

Should this just use the image's load event?

[Łukasz Anforowicz, 2016/04/25 23:33:50]

Done.  You're right, the layoutAndPaintAsyncThen is wrong - I should have used
the load event.

+                testRunner.notifyDone();
+            });
         }
     </script>
 </body>
-</html
+</html>