Add metric with time between background tab being foregrounded and the first paint

components/page_load_metrics/browser/page_load_metrics_observer.h

Index: components/page_load_metrics/browser/page_load_metrics_observer.h
diff --git a/components/page_load_metrics/browser/page_load_metrics_observer.h b/components/page_load_metrics/browser/page_load_metrics_observer.h
index 06705b20fe179ce8d258049929dde4531beb9554..6f68b84fbbb36e463f539da7326ad66073349107 100644
--- a/components/page_load_metrics/browser/page_load_metrics_observer.h
+++ b/components/page_load_metrics/browser/page_load_metrics_observer.h
@@ -55,14 +55,12 @@ struct PageLoadExtraInfo {
                     UserAbortType abort_type,
                     const base::TimeDelta& time_to_abort);
 
-  // Returns the time to first background if the page load started in the
-  // foreground. If the page has not been backgrounded, or the page started in
-  // the background, this will be base::TimeDelta().
+  // The first time that the page was backgrounded since the navigation started.
+  // If the page has not been backgrounded this will be base::TimeDelta().
   const base::TimeDelta first_background_time;
 
-  // Returns the time to first foreground if the page load started in the
-  // background. If the page has not been foregrounded, or the page started in
-  // the foreground, this will be base::TimeDelta().
+  // The first time that the page was foregrounded since the navigation started.
+  // If the page has not been foregrounded this will be base::TimeDelta().
   const base::TimeDelta first_foreground_time;
 
   // True if the page load started in the foreground.