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 0f455c237d663baff495ba31a6c312996d448a8a..c42f36d93ee467eab1375a7bad8abbb8b90a4441 100644
--- a/components/page_load_metrics/browser/page_load_metrics_observer.h
+++ b/components/page_load_metrics/browser/page_load_metrics_observer.h
@@ -59,14 +59,12 @@ struct PageLoadExtraInfo {
                     UserAbortType abort_type,
                     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.