Add immediate variants of doc.write page load metrics

components/page_load_metrics/browser/page_load_metrics_observer.h

Index: components/page_load_metrics/browser/page_load_metrics_observer.h
diff --git a/components/page_load_metrics/browser/page_load_metrics_observer.h b/components/page_load_metrics/browser/page_load_metrics_observer.h
index 796a3678ce8a12b490c55192a38262a9f00aa46d..a08ec0ad45c50daa923777d3e9020b7c919bca7c 100644
--- a/components/page_load_metrics/browser/page_load_metrics_observer.h
+++ b/components/page_load_metrics/browser/page_load_metrics_observer.h
@@ -179,6 +179,11 @@ class PageLoadMetricsObserver {
                             const PageLoadExtraInfo& extra_info) {}
   virtual void OnParseStop(const PageLoadTiming& timing,
                            const PageLoadExtraInfo& extra_info) {}
+
+  // Observer method to be invoked when there is a change in PageLoadMetadata's
+  // behavior_flags.
+  virtual void OnLoadingBehaviorObserved(
+      const page_load_metrics::PageLoadExtraInfo& extra_info) {}
 };
 
 }  // namespace page_load_metrics