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 309685452a9246bb1e0ff5d4d38f2e13721194b1..9b14bef9e1d04f7f85e3784e421422fc3b0dc93e 100644
--- a/chrome/browser/page_load_metrics/metrics_web_contents_observer.h
+++ b/chrome/browser/page_load_metrics/metrics_web_contents_observer.h
@@ -75,6 +75,12 @@ 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(