Deflake external/wpt/infrastructure/testdriver/click_iframe_crossorigin.sub.html

This test can flake if the iframe is loaded OOP and the child script
runs before the parent script.

Bug: 1137228
Change-Id: I13b8d2d85cae6d0eb5ab00029a388ea321c86150
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2481347
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819340}

third_party/blink/web_tests/external/wpt/infrastructure/testdriver/click_iframe_crossorigin.sub.html

@@ -6,8 +6,6 @@
 <script src="/resources/testdriver.js"></script>
 <script src="/resources/testdriver-vendor.js"></script>
 
-<iframe src="https://{{host}}:{{ports[https][1]}}/infrastructure/testdriver/click_child_testdriver.html"></iframe>
-
 <script>
 setup({single_test: true});
 addEventListener("message", (msg) => {
@@ -18,3 +16,8 @@ addEventListener("message", (msg) => {
     }
 });
 </script>
+
+<!-- Make sure we add the event listener before loading the iframe, to avoid
+potentially missing messages from the child. -->
+<iframe src="https://{{host}}:{{ports[https][1]}}/infrastructure/testdriver/click_child_testdriver.html"></iframe>
+