Remove the line-number flakiness by adding an img element dynamically.

third_party/WebKit/LayoutTests/http/tests/security/mixedContent/resources/frame-with-insecure-srcset.html

Index: third_party/WebKit/LayoutTests/http/tests/security/mixedContent/resources/frame-with-insecure-srcset.html
diff --git a/third_party/WebKit/LayoutTests/http/tests/security/mixedContent/resources/frame-with-insecure-srcset.html b/third_party/WebKit/LayoutTests/http/tests/security/mixedContent/resources/frame-with-insecure-srcset.html
index e75e59a7262019ffc50f397a44de054c6d3f560d..8b3b100efca161695ff7e00d3c484e9b37f75c4c 100644
--- a/third_party/WebKit/LayoutTests/http/tests/security/mixedContent/resources/frame-with-insecure-srcset.html
+++ b/third_party/WebKit/LayoutTests/http/tests/security/mixedContent/resources/frame-with-insecure-srcset.html
@@ -1,8 +1,12 @@
-<img src="http://127.0.0.1:8080/security/resources/compass.jpg"
-     srcset="http://127.0.0.1:8080/security/resources/compass.jpg 2x">
 <script>
 window.onload = function() {
-    if (window.opener)
-        window.opener.postMessage('done', '*');
+    var img = document.createElement('img');
+    img.src = "http://127.0.0.1:8080/security/resources/compass.jpg";
+    img.srcset = "http://127.0.0.1:8080/security/resources/compass.jpg 2x";
+    img.addEventListener('load', function () {
+        if (window.opener)
+            window.opener.postMessage('done', '*');
+    });
+    document.body.appendChild(img);
 };
 </script>