Plumb experiment flags through page_load_metrics and add a new observer

chrome/browser/page_load_metrics/page_load_metrics_initialize.cc

 // Copyright 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 "chrome/browser/page_load_metrics/page_load_metrics_initialize.h"
 
 #include "chrome/browser/page_load_metrics/observers/aborts_page_load_metrics_observer.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/browser/page_load_metrics/observers/from_gws_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/google_captcha_observer.h"
 #include "chrome/browser/page_load_metrics/observers/stale_while_revalidate_metrics_observer.h"
 #include "chrome/browser/prerender/prerender_contents.h"
 #include "components/page_load_metrics/browser/metrics_web_contents_observer.h"
 #include "components/rappor/rappor_service.h"
 #include "content/public/browser/web_contents.h"
 
 namespace chrome {
 
 void InitializePageLoadMetricsForWebContents(
     content::WebContents* web_contents) {
   page_load_metrics::MetricsWebContentsObserver::CreateForWebContents(
       web_contents, make_scoped_ptr(new PageLoadMetricsEmbedder()));
 }
 
 PageLoadMetricsEmbedder::~PageLoadMetricsEmbedder() {}
 
 void PageLoadMetricsEmbedder::RegisterObservers(
     page_load_metrics::PageLoadTracker* tracker) {
   // These classes are owned by the metrics.
   tracker->AddObserver(make_scoped_ptr(new AbortsPageLoadMetricsObserver()));
   tracker->AddObserver(make_scoped_ptr(new CorePageLoadMetricsObserver()));
   tracker->AddObserver(make_scoped_ptr(new FromGWSPageLoadMetricsObserver()));
   tracker->AddObserver(
       make_scoped_ptr(new google_captcha_observer::GoogleCaptchaObserver()));
   // TODO(ricea): Remove this in April 2016 or before. crbug.com/348877
   tracker->AddObserver(
       make_scoped_ptr(new chrome::StaleWhileRevalidateMetricsObserver()));
+  tracker->AddObserver(
+      make_scoped_ptr(new DocumentWritePageLoadMetricsObserver()));
 }
 
 bool PageLoadMetricsEmbedder::IsPrerendering(
     content::WebContents* web_contents) {
   return prerender::PrerenderContents::FromWebContents(web_contents) != nullptr;
 }
 
 }  // namespace chrome