Auto merge of #19528 - asajeffrey:test-perf-measure-loadEventEnd, r=avadacatavra

Capture loadEventEnd in test-perf

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Capture perf-test data after the load event has finished firing.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because this is test infrastructure

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etc/ci/performance/user-agent-js/01.perf-timing.js

@@ -41,7 +41,9 @@ if (document.readyState === "complete") {
     printPerfTiming()
     window.close();
 } else {
-    window.addEventListener('load', printPerfTiming);
+    window.addEventListener('load', function () {
+	window.setTimeout(printPerfTiming, 0);
+    });
     var timeout = 5;
     window.setTimeout(function() {
         print("[PERF] Timeout after " + timeout + " min. Force stop");