servo: Merge #19528 - Capture loadEventEnd in test-perf (from asajeffrey:test-perf-measure-loadEventEnd); r=avadacatavra

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

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [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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Source-Repo: https://github.com/servo/servo
Source-Revision: 89f9af88a31e4b07609658545f1c2bf6878a2acc

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 27f95833b3a94cb3f8e1f5e1e658721634932ad5

servo/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");