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

chrome/browser/page_load_metrics/metrics_web_contents_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_METRICS_WEB_CONTENTS_OBSERVER_H_
 #define CHROME_BROWSER_PAGE_LOAD_METRICS_METRICS_WEB_CONTENTS_OBSERVER_H_
 
 #include <map>
 #include <memory>
 #include <vector>
@@ skipping 56 matching lines @@
                              content::RenderViewHost* new_host) override;
   void MediaStartedPlaying(
       const content::WebContentsObserver::MediaPlayerInfo& video_type,
       const content::WebContentsObserver::MediaPlayerId& id) override;
 
   // These methods are forwarded from the MetricsNavigationThrottle.
   void WillStartNavigationRequest(content::NavigationHandle* navigation_handle);
   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,
                          content::ResourceType resource_type,
                          bool was_cached,
                          bool used_data_reduction_proxy,
                          int64_t raw_body_bytes,
                          int64_t original_content_length,
                          base::TimeTicks creation_time);
 
@@ skipping 78 matching lines @@
 
   // Has the MWCO observed at least one navigation?
   bool has_navigated_;
 
   DISALLOW_COPY_AND_ASSIGN(MetricsWebContentsObserver);
 };
 
 }  // namespace page_load_metrics
 
 #endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_METRICS_WEB_CONTENTS_OBSERVER_H_