[LargestContentfulPaint] Add experimental LCP for AMP

UKM privacy review:
https://docs.google.com/document/d/1M1nsLapTIoNgaD8SXtrqEirVQajYKmfmQKHbtI7J6-w/edit#heading=h.k5jx6iluw4yt

Bug: 1045640
Change-Id: I09e2e779f28d42e3760ed1acaf08c7614e3f9194
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450909Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814700}

chrome/browser/page_load_metrics/observers/core/amp_page_load_metrics_observer.cc

@@ -401,6 +401,15 @@ void AMPPageLoadMetricsObserver::MaybeRecordAmpDocumentMetrics() {
             largest_content_paint_time.value());
       }
     }
+    if (page_load_metrics::LargestContentfulPaintHandler::
+            AssignTimeAndSizeForLargestContentfulPaint(
+                *paint_timing->experimental_largest_contentful_paint,
+                &largest_content_paint_time, &largest_content_paint_size,
+                &largest_content_type)) {
+      builder
+          .SetSubFrame_PaintTiming_NavigationToExperimentalLargestContentfulPaint(
+              largest_content_paint_time.value().InMilliseconds());
+    }
 
     if (subframe_info.timing->interactive_timing->first_input_delay
             .has_value()) {

chrome/browser/page_load_metrics/observers/core/amp_page_load_metrics_observer_unittest.cc

@@ -332,6 +332,10 @@ TEST_F(AMPPageLoadMetricsObserverTest, SubFrameMetrics) {
       ->largest_image_paint_size = 1;
   subframe_timing.paint_timing->largest_contentful_paint->largest_image_paint =
       base::TimeDelta::FromMilliseconds(10);
+  subframe_timing.paint_timing->experimental_largest_contentful_paint
+      ->largest_text_paint_size = 3;
+  subframe_timing.paint_timing->experimental_largest_contentful_paint
+      ->largest_text_paint = base::TimeDelta::FromMilliseconds(8);
   subframe_timing.interactive_timing->first_input_timestamp =
       base::TimeDelta::FromMilliseconds(20);
   subframe_timing.interactive_timing->first_input_delay =
@@ -366,6 +370,9 @@ TEST_F(AMPPageLoadMetricsObserverTest, SubFrameMetrics) {
   tester()->test_ukm_recorder().ExpectEntryMetric(
       entry.get(), "SubFrame.PaintTiming.NavigationToLargestContentfulPaint",
       10);
+  tester()->test_ukm_recorder().ExpectEntryMetric(
+      entry.get(),
+      "SubFrame.PaintTiming.NavigationToExperimentalLargestContentfulPaint", 8);
 }
 
 TEST_F(AMPPageLoadMetricsObserverTest, SubFrameMetrics_LayoutInstability) {
@@ -440,6 +447,10 @@ TEST_F(AMPPageLoadMetricsObserverTest, SubFrameMetricsFullNavigation) {
       ->largest_image_paint_size = 1;
   subframe_timing.paint_timing->largest_contentful_paint->largest_image_paint =
       base::TimeDelta::FromMilliseconds(10);
+  subframe_timing.paint_timing->experimental_largest_contentful_paint
+      ->largest_image_paint_size = 1;
+  subframe_timing.paint_timing->experimental_largest_contentful_paint
+      ->largest_image_paint = base::TimeDelta::FromMilliseconds(5);
   subframe_timing.interactive_timing->first_input_timestamp =
       base::TimeDelta::FromMilliseconds(20);
   subframe_timing.interactive_timing->first_input_delay =
@@ -475,6 +486,9 @@ TEST_F(AMPPageLoadMetricsObserverTest, SubFrameMetricsFullNavigation) {
   tester()->test_ukm_recorder().ExpectEntryMetric(
       entry.get(), "SubFrame.PaintTiming.NavigationToLargestContentfulPaint",
       10);
+  tester()->test_ukm_recorder().ExpectEntryMetric(
+      entry.get(),
+      "SubFrame.PaintTiming.NavigationToExperimentalLargestContentfulPaint", 5);
 }
 
 TEST_F(AMPPageLoadMetricsObserverTest, SubFrameRecordOnFullNavigation) {

tools/metrics/ukm/ukm.xml

@@ -440,6 +440,17 @@ be describing additional metrics about the same event.
       was not prerendered.
     </summary>
   </metric>
+  <metric
+      name="SubFrame.PaintTiming.NavigationToExperimentalLargestContentfulPaint">
+    <summary>
+      Measures the time in milliseconds from navigation timing's navigation
+      start to the time when the page first paints the experimental largest
+      content (text or image) within viewport, in the AMP subframe. See
+      http://bit.ly/largest_contentful_paint_explainer for more details. Differs
+      from the non-experimental version in that removed content is still
+      considered a valid candidate.
+    </summary>
+  </metric>
   <metric name="SubFrame.PaintTiming.NavigationToFirstContentfulPaint">
     <summary>
       Measures the time in milliseconds from navigation timing's navigation