Don't log background navigations in StaleWhileRevalidateExperiment histograms

chrome/browser/page_load_metrics/observers/stale_while_revalidate_metrics_observer.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/observers/stale_while_revalidate_metrics_observer.h"
 
 #include "components/page_load_metrics/browser/page_load_metrics_util.h"
 #include "components/page_load_metrics/common/page_load_timing.h"
 #include "content/public/browser/navigation_handle.h"
 #include "net/base/stale_while_revalidate_experiment_domains.h"
 
 namespace chrome {
 
 StaleWhileRevalidateMetricsObserver::StaleWhileRevalidateMetricsObserver(
     page_load_metrics::PageLoadMetricsObservable* metrics)
     : is_interesting_domain_(false), metrics_(metrics) {}
 
 void StaleWhileRevalidateMetricsObserver::OnCommit(
     content::NavigationHandle* navigation_handle) {
   is_interesting_domain_ = net::IsHostInStaleWhileRevalidateExperimentDomain(
       navigation_handle->GetURL().host());
 }
 
 void StaleWhileRevalidateMetricsObserver::OnComplete(
     const page_load_metrics::PageLoadTiming& timing,
     const page_load_metrics::PageLoadExtraInfo& extra_info) {
+  using page_load_metrics::EventOccurredInForeground;
+
   if (!is_interesting_domain_)
     return;
 
-  if (!timing.load_event_start.is_zero()) {
+  // A page load that started in the background usually means a prerender. The
+  // performance metrics for a prerender aren't directly comparable to normal
+  // foreground page loads, so just drop them.
+  if (!extra_info.started_in_foreground)
+    return;
+
+  const base::TimeDelta& first_background = extra_info.first_background_time;
+
+  if (EventOccurredInForeground(timing.load_event_start, first_background)) {
     PAGE_LOAD_HISTOGRAM(
         "PageLoad.Clients.StaleWhileRevalidateExperiment.Timing2."
         "NavigationToLoadEventFired",
         timing.load_event_start);
   }
-  if (!timing.first_layout.is_zero()) {
+  if (EventOccurredInForeground(timing.first_layout, first_background)) {
     PAGE_LOAD_HISTOGRAM(
         "PageLoad.Clients.StaleWhileRevalidateExperiment.Timing2."
         "NavigationToFirstLayout",
         timing.first_layout);
   }
-  if (!timing.first_text_paint.is_zero()) {
+  if (EventOccurredInForeground(timing.first_text_paint, first_background)) {
     PAGE_LOAD_HISTOGRAM(
         "PageLoad.Clients.StaleWhileRevalidateExperiment.Timing2."
         "NavigationToFirstTextPaint",
         timing.first_text_paint);
   }
 }
 
 void StaleWhileRevalidateMetricsObserver::OnPageLoadMetricsGoingAway() {
   metrics_->RemoveObserver(this);
   delete this;
 }
 
 }  // namespace chrome