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

chrome/browser/page_load_metrics/page_load_metrics_observer.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_PAGE_LOAD_METRICS_PAGE_LOAD_METRICS_OBSERVER_H_
 #define CHROME_BROWSER_PAGE_LOAD_METRICS_PAGE_LOAD_METRICS_OBSERVER_H_
 
 #include <string>
 
 #include "base/macros.h"
@@ skipping 351 matching lines @@
   // frequently on Android).
   virtual void OnComplete(const PageLoadTiming& timing,
                           const PageLoadExtraInfo& extra_info) {}
 
   // OnFailedProvisionalLoad is invoked for tracked page loads that did not
   // commit, immediately before the observer is deleted.
   virtual void OnFailedProvisionalLoad(
       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) {}
 };
 
 }  // namespace page_load_metrics
 
 #endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_PAGE_LOAD_METRICS_OBSERVER_H_