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

components/page_load_metrics/browser/page_load_metrics_observer.h

 // 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.
 
 #ifndef COMPONENTS_PAGE_LOAD_METRICS_BROWSER_PAGE_LOAD_METRICS_OBSERVER_H_
 #define COMPONENTS_PAGE_LOAD_METRICS_BROWSER_PAGE_LOAD_METRICS_OBSERVER_H_
 
 #include "base/macros.h"
 #include "components/page_load_metrics/common/page_load_timing.h"
 #include "content/public/browser/navigation_handle.h"
@@ skipping 41 matching lines @@
 
 struct PageLoadExtraInfo {
   PageLoadExtraInfo(base::TimeDelta first_background_time,
                     base::TimeDelta first_foreground_time,
                     bool started_in_foreground,
                     const GURL& committed_url,
                     base::TimeDelta time_to_commit,
                     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.
   const bool started_in_foreground;
 
   // Committed URL. If the page load did not commit, |committed_url| will be
   // empty.
   const GURL committed_url;
 
   // Time from navigation start until commit. If the page load did not commit,
@@ skipping 42 matching lines @@
   // data collected over the course of the page load. If the load did not
   // receive any timing information, |timing.IsEmpty()| will be true.
   // After this call, the object will be deleted.
   virtual void OnComplete(const PageLoadTiming& timing,
                           const PageLoadExtraInfo& extra_info) {}
 };
 
 }  // namespace page_load_metrics
 
 #endif  // COMPONENTS_PAGE_LOAD_METRICS_BROWSER_PAGE_LOAD_METRICS_OBSERVER_H_