Unflake dynamic adding of preload links test

third_party/WebKit/LayoutTests/http/tests/preload/dynamic_adding_preload.html

Index: third_party/WebKit/LayoutTests/http/tests/preload/dynamic_adding_preload.html
diff --git a/third_party/WebKit/LayoutTests/http/tests/preload/dynamic_adding_preload.html b/third_party/WebKit/LayoutTests/http/tests/preload/dynamic_adding_preload.html
index 21b0a9bfd6e2df710d46246be3f10ce3a94c2abb..5d07d5d906ed002613271288fd27b7b1aef80c90 100644
--- a/third_party/WebKit/LayoutTests/http/tests/preload/dynamic_adding_preload.html
+++ b/third_party/WebKit/LayoutTests/http/tests/preload/dynamic_adding_preload.html
@@ -10,11 +10,10 @@
     link.as = "script";
     link.rel = "preload";
     link.href = "../resources/dummy.js";
-    document.body.appendChild(link);
-    window.addEventListener("load", t.step_func(function() {
-        assert_equals(performance.getEntriesByType("resource").length, 4);
+    link.onload = t.step_func(function() {
+        assert_equals(performance.getEntriesByType("resource").length, 3);
         t.done();
-    }));
+    });
+    document.body.appendChild(link);
 </script>
-<script src="../resources/slow-script.pl?delay=200"></script>
 </body>