Log first meaningful paint for the document.write evaluator

chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc

Index: chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc b/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc
index 433b7e59a3bb566d108b731f646ace0d2c70a742..b0cea165207eb522b415f2973149e1d702bdd399 100644
--- a/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc
+++ b/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc
@@ -145,6 +145,15 @@ void DocumentWritePageLoadMetricsObserver::OnFirstContentfulPaint(
   }
 }
 
+void DocumentWritePageLoadMetricsObserver::OnFirstMeaningfulPaint(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& info) {
+  if (info.metadata.behavior_flags &
+      blink::WebLoadingBehaviorFlag::WebLoadingBehaviorDocumentWriteEvaluator) {
+    LogDocumentWriteEvaluatorFirstMeaningfulPaint(timing, info);
+  }
+}
+
 void DocumentWritePageLoadMetricsObserver::OnParseStop(
     const page_load_metrics::PageLoadTiming& timing,
     const page_load_metrics::PageLoadExtraInfo& info) {
@@ -207,6 +216,25 @@ void DocumentWritePageLoadMetricsObserver::
   }
 }
 
+void DocumentWritePageLoadMetricsObserver::
+    LogDocumentWriteEvaluatorFirstMeaningfulPaint(
+        const page_load_metrics::PageLoadTiming& timing,
+        const page_load_metrics::PageLoadExtraInfo& info) {
+  // Note: The first meaningful paint calculation in the core observer filters
+  // out pages which had user interaction before the first meaningful paint.
+  // Because the counts of those instances are low (< 2%), just log everything
+  // here for simplicity. If this ends up being unreliable (the 2% is just from
+  // canary), the page_load_metrics API should be altered to return the values
+  // the consumer wants.
+  if (WasStartedInForegroundOptionalEventInForeground(
+          timing.first_meaningful_paint, info)) {
+    PAGE_LOAD_HISTOGRAM(
+        "PageLoad.Clients.DocWrite.Evaluator.Experimental.PaintTiming."
+        "ParseStartToFirstMeaningfulPaint",
+        timing.first_meaningful_paint.value() - timing.parse_start.value());
+  }
+}
+
 void DocumentWritePageLoadMetricsObserver::LogDocumentWriteEvaluatorParseStop(
     const page_load_metrics::PageLoadTiming& timing,
     const page_load_metrics::PageLoadExtraInfo& info) {