Plumb experiment flags through page_load_metrics and add a new observer

chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h

Index: chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h
diff --git a/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h b/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..a24421b2e7a2fed61f9494562618d791f1c18bf3
--- /dev/null
+++ b/chrome/browser/page_load_metrics/observers/document_write_page_load_metrics_observer.h
@@ -0,0 +1,35 @@
+// 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.
+
+#ifndef CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_DOCUMENT_WRITE_PAGE_LOAD_METRICS_OBSERVER_H_
+#define CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_DOCUMENT_WRITE_PAGE_LOAD_METRICS_OBSERVER_H_
+
+#include "base/macros.h"
+#include "components/page_load_metrics/browser/page_load_metrics_observer.h"
+
+namespace internal {
+
+// Just expose one metric for tests.
+extern const char kHistogramDocWriteParseStartToFirstContentfulPaint[];
+
+}  // namespace internal
+
+class DocumentWritePageLoadMetricsObserver
+    : public page_load_metrics::PageLoadMetricsObserver {
+ public:
+  DocumentWritePageLoadMetricsObserver();
+  // page_load_metrics::PageLoadMetricsObserver implementation:
+  void OnComplete(
+      const page_load_metrics::PageLoadTiming& timing,
+      const page_load_metrics::PageLoadExtraInfo& extra_info) override;
+
+ private:
+  void LogDocumentWriteEvaluatorData(
+      const page_load_metrics::PageLoadTiming& timing,
+      const page_load_metrics::PageLoadExtraInfo& info);
+
+  DISALLOW_COPY_AND_ASSIGN(DocumentWritePageLoadMetricsObserver);
+};
+
+#endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_DOCUMENT_WRITE_PAGE_LOAD_METRICS_OBSERVER_H_