Plumb experiment flags through page_load_metrics and add a new observer

chrome/browser/page_load_metrics/page_load_metrics_browsertest.cc

 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #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"
 
 class MetricsWebContentsObserverBrowserTest : public InProcessBrowserTest {
  public:
   MetricsWebContentsObserverBrowserTest() {}
   ~MetricsWebContentsObserverBrowserTest() override {}
 
  protected:
@@ skipping 30 matching lines @@
                                embedded_test_server()->GetURL("/title1.html"));
   ui_test_utils::NavigateToURL(
       browser(), embedded_test_server()->GetURL("/title1.html#hash"));
   ui_test_utils::NavigateToURL(browser(),
                                embedded_test_server()->GetURL("/title2.html"));
 
   histogram_tester_.ExpectTotalCount(internal::kHistogramDomContentLoaded, 1);
   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::kHistogramDocWriteDomLoadingToFirstContentfulPaint, 1);
+}
+
+IN_PROC_BROWSER_TEST_F(MetricsWebContentsObserverBrowserTest,
+                       NoPreloadDocumentWrite) {
+  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::kHistogramDocWriteDomLoadingToFirstContentfulPaint, 0);
+}