Send an event to the page load metrics to track resource starting.

chrome/browser/page_load_metrics/metrics_web_contents_observer.h

Index: chrome/browser/page_load_metrics/metrics_web_contents_observer.h
diff --git a/chrome/browser/page_load_metrics/metrics_web_contents_observer.h b/chrome/browser/page_load_metrics/metrics_web_contents_observer.h
index f9e958702bcd34efc90548c611844af484391cc6..8c9335d9421eabb9cca16e655f25eeff8601aac7 100644
--- a/chrome/browser/page_load_metrics/metrics_web_contents_observer.h
+++ b/chrome/browser/page_load_metrics/metrics_web_contents_observer.h
@@ -74,6 +74,12 @@ class MetricsWebContentsObserver
   void WillProcessNavigationResponse(
       content::NavigationHandle* navigation_handle);
 
+  // A resource request started on the IO thread. This method is invoked on
+  // the UI thread.
+  void OnRequestStarted(const content::GlobalRequestID& request_id,
+                        content::ResourceType resource_type,
+                        base::TimeTicks creation_time);
+
   // A resource request completed on the IO thread. This method is invoked on
   // the UI thread.
   void OnRequestComplete(const content::GlobalRequestID& request_id,