Log an immediate variant of FirstContentfulPaint.

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 5bee249f8d0c38955541e728e2857adfe60b02fd..48ef2cb4906e64049a10fcb9acc5e7af8705ac8e 100644
--- a/components/page_load_metrics/browser/page_load_metrics_observer.h
+++ b/components/page_load_metrics/browser/page_load_metrics_observer.h
@@ -125,6 +125,14 @@ class PageLoadMetricsObserver {
   virtual void OnFailedProvisionalLoad(
       content::NavigationHandle* navigation_handle) {}
 
+  // OnTimingUpdate is triggered when an updated PageLoadTiming is
+  // available. This method may be called multiple times over the course of the
+  // page load. Note that this is currently an experimental API which may be
+  // removed in the future. Please email loading-dev@chromium.org if you intend
+  // to override this method.
+  virtual void OnTimingUpdate(const PageLoadTiming& timing,
+                              const PageLoadExtraInfo& extra_info) {}
+
   // OnComplete is triggered when we are ready to record metrics for this page
   // load. This will happen some time after commit. The PageLoadTiming struct
   // contains timing data and the PageLoadExtraInfo struct contains other useful