Don't log background navigations in StaleWhileRevalidateExperiment histograms

chrome/browser/page_load_metrics/observers/stale_while_revalidate_metrics_observer.cc

Index: chrome/browser/page_load_metrics/observers/stale_while_revalidate_metrics_observer.cc
diff --git a/chrome/browser/page_load_metrics/observers/stale_while_revalidate_metrics_observer.cc b/chrome/browser/page_load_metrics/observers/stale_while_revalidate_metrics_observer.cc
index cfa4fbb00bcdf5a02616d717ff398ae35525741e..2162a6300a3e129b32b4a17940af6ed92e183c2a 100644
--- a/chrome/browser/page_load_metrics/observers/stale_while_revalidate_metrics_observer.cc
+++ b/chrome/browser/page_load_metrics/observers/stale_while_revalidate_metrics_observer.cc
@@ -24,22 +24,32 @@ void StaleWhileRevalidateMetricsObserver::OnCommit(
 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",