Bug 1497681 [wpt PR 13437] - Fix flakiness in resource_timing_buffer_full_250, a=testonly

Automatic update from web-platform-testsFix flakiness in resource_timing_buffer_full_250

This CL uses document fragments to append scripts faster. Also, we do
not need to wait until onload of one script to begin appending the next.

Bug: 626703, 883837
Change-Id: I0bc010640793d6a7113d404f2838fd5cfe88ab25
Reviewed-on: https://chromium-review.googlesource.com/c/1265901
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598896}

--

wpt-commits: ed3b742cd294a5d39d40bc4f8271e9ebe864245f
wpt-pr: 13437

testing/web-platform/tests/resource-timing/resource_timing_buffer_full_eventually.html

@@ -16,15 +16,23 @@
         function() {
             // Scripts appended in JS to ensure setResourceTimingBufferSize is called before.
             let counter = performance.getEntriesByType("resource").length;
-            function appendScript() {
+            function appendScripts() {
+                const documentFragment = document.createDocumentFragment();
+                // Add 100 elements at a time to avoid page reflow every time.
+                let numScriptsAccumulated = 0;
+                while (numScriptsAccumulated < 100) {
                     const src = "resources/empty.js?" + counter;
                     const script = document.createElement('script');
                     script.type = 'text/javascript';
-                script.onload = function() { ++counter; appendScript()};
                     script.src = src;
-                document.body.appendChild(script);
+                    documentFragment.appendChild(script);
+                    ++counter;
+                    ++numScriptsAccumulated;
                 }
-            appendScript();
+                document.body.appendChild(documentFragment);
+                t.step_timeout(appendScripts, 20);
+            }
+            appendScripts();
         });
 </script>
 </body>