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

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 41acd9cd0f165b1e0b712c63f3a74a0f2e658d18..b12854824ba0efd4bccc2d380e749b74b631f839 100644
--- a/chrome/browser/page_load_metrics/page_load_metrics_observer.h
+++ b/chrome/browser/page_load_metrics/page_load_metrics_observer.h
@@ -369,6 +369,9 @@ class PageLoadMetricsObserver {
       const FailedProvisionalLoadInfo& failed_provisional_load_info,
       const PageLoadExtraInfo& extra_info) {}
 
+  // Called whenever a request load begins.
+  virtual void OnStartedResource(const ExtraRequestInfo& extra_request_info) {}

[RyanSturm, 2017/03/29 19:03:47]

I'd recommend adding an actual observer (and tests) in the same CL.

[Pete Williamson, 2017/04/13 23:10:21]

Done.

+
   // Called whenever a request is loaded for this page load.
   virtual void OnLoadedResource(const ExtraRequestInfo& extra_request_info) {}
 };