Adding previews information to PLM UKM

chrome/browser/page_load_metrics/page_load_metrics_observer.h

Index: chrome/browser/page_load_metrics/page_load_metrics_observer.h
diff --git a/chrome/browser/page_load_metrics/page_load_metrics_observer.h b/chrome/browser/page_load_metrics/page_load_metrics_observer.h
index 3f964a23fc826bed29fcd3397472ea0d3c136ae0..459bddab9e11c6d85cd19dd7ff02a90b91e26b32 100644
--- a/chrome/browser/page_load_metrics/page_load_metrics_observer.h
+++ b/chrome/browser/page_load_metrics/page_load_metrics_observer.h
@@ -451,6 +451,10 @@ class PageLoadMetricsObserver {
   // and data uris.
   virtual void OnLoadedResource(
       const ExtraRequestCompleteInfo& extra_request_complete_info) {}
+
+  // Called when the event corresponding to |event_key| occurs in this page
+  // load.
+  virtual void OnEventOccurred(const void* const event_key) {}
 };
 
 }  // namespace page_load_metrics