Unflake loaf test

- loaf-buffered test: don't assume that the first LoAF entry is
the one we injected.

Bug: 1420756
Change-Id: Id9ba50d0b24b3cfa42c21f01fb45a5c808700352
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4302731
Commit-Queue: Noam Rosenthal <nrosenthal@chromium.org>
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1112110}

long-animation-frame/tentative/loaf-buffered.html

@@ -13,13 +13,16 @@ <h1>Long Animation Frame: buffered</h1>
 promise_test(async t => {
     busy_wait(very_long_frame_duration);
     await new Promise(resolve => t.step_timeout(resolve, 0));
-    const entry = await new Promise(resolve => {
-        new PerformanceObserver(t.step_func((entryList) => {
-            const entry = entryList.getEntries()[0];
-            if (entry.duration >= very_long_frame_duration)
-                resolve(entry);
+    const result = await new Promise(resolve => {
+        new PerformanceObserver(t.step_func(entries => {
+            for (const e of entries.getEntries()) {
+                if (e.duration >= very_long_frame_duration)
+                    resolve("entry-found");
+            }
         })).observe({type: 'long-animation-frame', buffered: true});
+        t.step_timeout(() => resolve("timeout"), no_long_frame_timeout);
     });
+    assert_equals(result, "entry-found");
 }, 'PerformanceObserver with buffered flag can see previous long-animation-frame entries.');
 </script>
 </body>