[PaintTiming] Add fcp-only buffered flag test

Relevant GitHub issue:
GoogleChrome/web-vitals#139

Change-Id: I2884dd4c3432920011e984f6a08ebaf6a2176b67

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

@@ -0,0 +1,27 @@
+setup({"hide_test_state": true});
+async_test(t => {
+  assert_implements(window.PerformancePaintTiming, "Paint Timing isn't supported.");
+  // First observer creates second in callback to ensure the entry has been dispatched by the time
+  // the second observer begins observing.
+  let entry_seen = false;
+  new PerformanceObserver(entries => {
+    entry_seen = entries.getEntriesByName('first-contentful-paint').length > 0;
+    // Abort if we have not yet received both paint entries.
+    if (!entry_seen)
+      return;
+
+    // Second observer requires 'buffered: true' to see the entries.
+    let firstContentfulPaintSeen = false;
+    new PerformanceObserver(list => {
+      let fcp = list.getEntriesByName('first-contentful-paint');
+      assert_equals(fcp.length, 1, 'Should have an fcp entry');
+      let entry = fcp[0];
+      assert_equals(entry.entryType, 'paint');
+      t.done();
+      }).observe({'type': 'paint', buffered: true});
+  }).observe({'entryTypes': ['paint']});
+  // Trigger the first paint entries
+  const img = document.createElement("IMG");
+  img.src = "resources/circles.png";
+  document.body.appendChild(img);
+}, "PerformanceObserver with buffered flag sees previous FCP entry.");