Add StaleWhileRevalidateExperiment histograms.

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/browser_process.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 {
 
 void RegisterPageLoadMetricsObservers(
     page_load_metrics::PageLoadMetricsObservable* metrics) {
   // Attach observers scoped to the web contents here.
 
   // This is a self-destruct class, and will delete itself when triggered by
   // OnPageLoadMetricsGoingAway.
   metrics->AddObserver(new FromGWSPageLoadMetricsObserver(metrics));
   metrics->AddObserver(
       new google_captcha_observer::GoogleCaptchaObserver(metrics));
+  // StaleWhileRevalidateMetricsObserver also deletes itself from
+  // OnPageLoadMetricsGoingAway.
+  // TODO(ricea): Remove this in April 2016 or before. crbug.com/348877
+  metrics->AddObserver(
+      new chrome::StaleWhileRevalidateMetricsObserver(metrics));
 }
 
 }  // namespace
 
 namespace chrome {
 
 void InitializePageLoadMetricsForWebContents(
     content::WebContents* web_contents) {
   RegisterPageLoadMetricsObservers(
       page_load_metrics::MetricsWebContentsObserver::CreateForWebContents(
           web_contents,
           make_scoped_ptr(new PageLoadMetricsEmbedderInterfaceImpl())));
 }
 
 PageLoadMetricsEmbedderInterfaceImpl::~PageLoadMetricsEmbedderInterfaceImpl() {}
 
 rappor::RapporService*
 PageLoadMetricsEmbedderInterfaceImpl::GetRapporService() {
   return g_browser_process->rappor_service();
 }
 
 bool PageLoadMetricsEmbedderInterfaceImpl::IsPrerendering(
     content::WebContents* web_contents) {
   return prerender::PrerenderContents::FromWebContents(web_contents) != nullptr;
 }
 
 }  // namespace chrome