Record the FirstPaint (FP) metric for subframes.

Change-Id: I2e817334fb9b8988dacbd21b531090dd4574359d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2265353Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782716}

chrome/browser/page_load_metrics/observers/amp_page_load_metrics_observer.cc

@@ -338,6 +338,12 @@ void AMPPageLoadMetricsObserver::MaybeRecordAmpDocumentMetrics() {
   }
 
   if (!subframe_info.timing.is_null()) {
+    if (subframe_info.timing->paint_timing->first_paint.has_value()) {
+      builder.SetSubFrame_PaintTiming_NavigationToFirstPaint(
+          subframe_info.timing->paint_timing->first_paint.value()
+              .InMilliseconds());
+    }
+
     if (subframe_info.timing->paint_timing->first_contentful_paint
             .has_value()) {
       builder.SetSubFrame_PaintTiming_NavigationToFirstContentfulPaint(

chrome/browser/page_load_metrics/observers/amp_page_load_metrics_observer_unittest.cc

@@ -324,6 +324,8 @@ TEST_F(AMPPageLoadMetricsObserverTest, SubFrameMetrics) {
   page_load_metrics::mojom::PageLoadTiming subframe_timing;
   page_load_metrics::InitPageLoadTimingForTest(&subframe_timing);
   subframe_timing.navigation_start = base::Time::FromDoubleT(2);
+  subframe_timing.paint_timing->first_paint =
+      base::TimeDelta::FromMilliseconds(4);
   subframe_timing.paint_timing->first_contentful_paint =
       base::TimeDelta::FromMilliseconds(5);
   subframe_timing.paint_timing->largest_contentful_paint
@@ -357,6 +359,8 @@ TEST_F(AMPPageLoadMetricsObserverTest, SubFrameMetrics) {
   tester()->test_ukm_recorder().ExpectEntrySourceHasUrl(entry.get(), amp_url);
   tester()->test_ukm_recorder().ExpectEntryMetric(
       entry.get(), "SubFrame.InteractiveTiming.FirstInputDelay4", 3);
+  tester()->test_ukm_recorder().ExpectEntryMetric(
+      entry.get(), "SubFrame.PaintTiming.NavigationToFirstPaint", 4);
   tester()->test_ukm_recorder().ExpectEntryMetric(
       entry.get(), "SubFrame.PaintTiming.NavigationToFirstContentfulPaint", 5);
   tester()->test_ukm_recorder().ExpectEntryMetric(

tools/metrics/ukm/ukm.xml

@@ -384,6 +384,12 @@ be describing additional metrics about the same event.
       A contentful paint includes a paint of text, image, SVG, or canvas.
     </summary>
   </metric>
+  <metric name="SubFrame.PaintTiming.NavigationToFirstPaint">
+    <summary>
+      Measures the time in milliseconds from navigation timing's navigation
+      start to the time the first paint is performed, in the AMP subframe.
+    </summary>
+  </metric>
   <metric name="SubFrame.PaintTiming.NavigationToLargestContentfulPaint">
     <summary>
       Measures the time in milliseconds from navigation timing's navigation