Add experiment to disable loading of background tabs during session restore.

chrome/browser/sessions/session_restore_stats_collector.h

Index: chrome/browser/sessions/session_restore_stats_collector.h
diff --git a/chrome/browser/sessions/session_restore_stats_collector.h b/chrome/browser/sessions/session_restore_stats_collector.h
index c312b404383a4530c356063b8acd35f0cd2ed9fd..24c622964e9598029c803c55bef83c6a2d7cb995 100644
--- a/chrome/browser/sessions/session_restore_stats_collector.h
+++ b/chrome/browser/sessions/session_restore_stats_collector.h
@@ -28,14 +28,16 @@ class SessionRestoreStatsCollector
   // are added to the existing list of tracked tabs.
   static void TrackTabs(
       const std::vector<SessionRestoreDelegate::RestoredTab>& tabs,
-      const base::TimeTicks& restore_started);
+      const base::TimeTicks& restore_started,
+      bool active_only);
 
  private:
   friend class base::RefCounted<SessionRestoreStatsCollector>;
 
   using RenderWidgetHostSet = std::set<content::RenderWidgetHost*>;
 
-  explicit SessionRestoreStatsCollector(const base::TimeTicks& restore_started);
+  explicit SessionRestoreStatsCollector(const base::TimeTicks& restore_started,
+                                        bool active_only);
   ~SessionRestoreStatsCollector() override;
 
   // NotificationObserver method.
@@ -83,6 +85,9 @@ class SessionRestoreStatsCollector
   // Notification registrar.
   content::NotificationRegistrar registrar_;
 
+  // Are we only restoring active tabs?
+  bool active_only_;
+
   // To keep the collector alive as long as needed.
   scoped_refptr<SessionRestoreStatsCollector> this_retainer_;