Plumb experiment flags through page_load_metrics and add a new observer

chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc

Index: chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
diff --git a/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc b/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
index c46c98b18c70b9cf9d5291a85aa55eaab0879112..0b8dee94c0573d3a26853bec968177f28b569e6c 100644
--- a/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
+++ b/chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc
@@ -5,6 +5,7 @@
 #include "base/macros.h"
 #include "base/test/histogram_tester.h"
 #include "chrome/browser/page_load_metrics/observers/core_page_load_metrics_observer.h"
+#include "chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h"
 #include "chrome/test/base/in_process_browser_test.h"
 #include "chrome/test/base/ui_test_utils.h"
 #include "net/test/embedded_test_server/embedded_test_server.h"
@@ -58,3 +59,42 @@ IN_PROC_BROWSER_TEST_F(MetricsWebContentsObserverBrowserTest, AnchorLink) {
   histogram_tester_.ExpectTotalCount(internal::kHistogramLoad, 1);
   histogram_tester_.ExpectTotalCount(internal::kHistogramFirstLayout, 1);
 }
+
+IN_PROC_BROWSER_TEST_F(MetricsWebContentsObserverBrowserTest,
+                       PreloadDocumentWrite) {
+  ASSERT_TRUE(embedded_test_server()->Start());
+
+  ui_test_utils::NavigateToURL(
+      browser(), embedded_test_server()->GetURL(
+                     "/page_load_metrics/document_write_external_script.html"));
+  ui_test_utils::NavigateToURL(browser(),
+                               embedded_test_server()->GetURL("/title2.html"));
+
+  histogram_tester_.ExpectTotalCount(
+      internal::kHistogramDocWriteParseStartToFirstContentfulPaint, 1);
+}
+
+IN_PROC_BROWSER_TEST_F(MetricsWebContentsObserverBrowserTest,
+                       NoPreloadDocumentWrite) {
+  ASSERT_TRUE(embedded_test_server()->Start());
+
+  ui_test_utils::NavigateToURL(
+      browser(), embedded_test_server()->GetURL(
+                     "/page_load_metrics/document_write_no_script.html"));
+  ui_test_utils::NavigateToURL(browser(),
+                               embedded_test_server()->GetURL("/title2.html"));
+
+  histogram_tester_.ExpectTotalCount(
+      internal::kHistogramDocWriteParseStartToFirstContentfulPaint, 0);
+}
+
+IN_PROC_BROWSER_TEST_F(MetricsWebContentsObserverBrowserTest, NoDocumentWrite) {
+  ASSERT_TRUE(embedded_test_server()->Start());
+
+  ui_test_utils::NavigateToURL(browser(),
+                               embedded_test_server()->GetURL("/title1.html"));
+  ui_test_utils::NavigateToURL(browser(),
+                               embedded_test_server()->GetURL("/title2.html"));
+  histogram_tester_.ExpectTotalCount(
+      internal::kHistogramDocWriteParseStartToFirstContentfulPaint, 0);
+}