[NoStatePrefetch] Add performance histograms.

chrome/browser/page_load_metrics/page_load_metrics_observer.h

Index: chrome/browser/page_load_metrics/page_load_metrics_observer.h
diff --git a/chrome/browser/page_load_metrics/page_load_metrics_observer.h b/chrome/browser/page_load_metrics/page_load_metrics_observer.h
index fb9e29f4e9993fd57f8b371d8462abe0f5d2bd51..9262a358aa96cea00330c86a143f83aa89708c66 100644
--- a/chrome/browser/page_load_metrics/page_load_metrics_observer.h
+++ b/chrome/browser/page_load_metrics/page_load_metrics_observer.h
@@ -67,6 +67,7 @@ struct PageLoadExtraInfo {
       bool started_in_foreground,
       bool user_gesture,
       const GURL& committed_url,
+      const GURL& start_url,
       const base::Optional<base::TimeDelta>& time_to_commit,
       UserAbortType abort_type,
       bool abort_user_initiated,
@@ -96,6 +97,9 @@ struct PageLoadExtraInfo {
   // empty.
   const GURL committed_url;
 
+  // The URL that started the navigation, before redirects.
+  const GURL start_url;
+
   // Time from navigation start until commit.
   const base::Optional<base::TimeDelta> time_to_commit;