Adding previews information to PLM UKM

chrome/browser/page_load_metrics/page_load_tracker.h

Index: chrome/browser/page_load_metrics/page_load_tracker.h
diff --git a/chrome/browser/page_load_metrics/page_load_tracker.h b/chrome/browser/page_load_metrics/page_load_tracker.h
index f8185b0f92359acbb1b58262f1f44803c58f2cfb..48e83ee549269b46409f4e25be73f20798636664 100644
--- a/chrome/browser/page_load_metrics/page_load_tracker.h
+++ b/chrome/browser/page_load_metrics/page_load_tracker.h
@@ -270,6 +270,8 @@ class PageLoadTracker : public PageLoadMetricsUpdateDispatcher::Client {
   void OnNavigationDelayComplete(base::TimeDelta scheduled_delay,
                                  base::TimeDelta actual_delay);
 
+  ukm::SourceId source_id() const { return source_id_; }
+
  private:
   // This function converts a TimeTicks value taken in the browser process
   // to navigation_start_ if: