Plumb experiment flags through page_load_metrics and add a new observer

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
new file mode 100644
index 0000000000000000000000000000000000000000..9ef040452b5f2d4142bb4c616e23068c27739f27
--- /dev/null
+++ b/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.cc
@@ -0,0 +1,36 @@
+// Copyright 2016 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 "chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h"
+#include "components/page_load_metrics/browser/page_load_metrics_util.h"
+#include "third_party/WebKit/public/platform/WebExperimentData.h"
+
+namespace internal {
+const char kHistogramDocWriteDomLoadingToFirstContentfulPaint[] =
+    "PageLoad.Clients.DocWrite.Evaluator.DOMLoadingToFirstContentfulPaint";
+}  // namespace internal
+
+DocumentWritePageLoadMetricsObserver::DocumentWritePageLoadMetricsObserver() {}
+
+void DocumentWritePageLoadMetricsObserver::OnComplete(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& info) {
+  if (info.time_to_commit.is_zero() || timing.IsEmpty())
+    return;
+  if (info.metadata.experiment_flags &
+      blink::WebExperimentData::DocumentWriteEvaluatorExperiment) {
+    LogDocumentWriteEvaluatorData(timing, info);
+  }
+}
+
+void DocumentWritePageLoadMetricsObserver::LogDocumentWriteEvaluatorData(
+    const page_load_metrics::PageLoadTiming& timing,
+    const page_load_metrics::PageLoadExtraInfo& info) {
+  if (WasStartedInForegroundEventInForeground(timing.first_contentful_paint,
+                                              info)) {
+    PAGE_LOAD_HISTOGRAM(
+        internal::kHistogramDocWriteDomLoadingToFirstContentfulPaint,
+        timing.first_contentful_paint - timing.dom_loading);
+  }
+}