[PaintTiming] Add fcp-only buffered flag test

Relevant GitHub issue:
GoogleChrome/web-vitals#139

Change-Id: I2884dd4c3432920011e984f6a08ebaf6a2176b67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2867865
Reviewed-by: Yoav Weiss <yoavweiss@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#878531}

paint-timing/fcp-only/buffered-flag.window.js

@@ -0,0 +1,24 @@
+setup({"hide_test_state": true});
+async_test(t => {
+  assert_implements(window.PerformancePaintTiming, "Paint Timing isn't supported.");
+  // First observer creates a second one in the callback to ensure the entry has been dispatched
+  // by the time the second observer begins observing.
+  new PerformanceObserver(entries => {
+    const entry_seen = entries.getEntriesByName('first-contentful-paint').length > 0;
+    // Abort if we have not yet received the entry.
+    if (!entry_seen)
+      return;
+
+    // Second observer requires 'buffered: true' to see the entry.
+    new PerformanceObserver(t.step_func_done(list => {
+        const fcp = list.getEntriesByName('first-contentful-paint');
+        assert_equals(fcp.length, 1, 'Should have an fcp entry');
+        const entry = fcp[0];
+        assert_equals(entry.entryType, 'paint');
+      })).observe({'type': 'paint', buffered: true});
+  }).observe({'entryTypes': ['paint']});
+  // Trigger the first contentful paint entry.
+  const img = document.createElement("img");
+  img.src = "resources/circles.png";
+  document.body.appendChild(img);
+}, "PerformanceObserver with buffered flag sees previous FCP entry.");