[Session restore] Add DontRestoreBackgroundTabs group to collect stats.

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..668e2c01b77a52e1a1ed57ea4017da9d870f44fd 100644
--- a/chrome/browser/sessions/session_restore_stats_collector.h
+++ b/chrome/browser/sessions/session_restore_stats_collector.h
@@ -30,6 +30,12 @@ class SessionRestoreStatsCollector
       const std::vector<SessionRestoreDelegate::RestoredTab>& tabs,
       const base::TimeTicks& restore_started);
 
+  // Called to start tracking only active tabs. If a restore is already
+  // occuring, the tabs are added to the existing list of tracked tabs.
+  static void TrackActiveTabs(
+      const std::vector<SessionRestoreDelegate::RestoredTab>& tabs,
+      const base::TimeTicks& restore_started);
+
  private:
   friend class base::RefCounted<SessionRestoreStatsCollector>;