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

chrome/browser/page_load_metrics/metrics_web_contents_observer.cc

 // 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.
 
 #include "chrome/browser/page_load_metrics/metrics_web_contents_observer.h"
 
 #include <algorithm>
 #include <ostream>
 #include <string>
 #include <utility>
@@ skipping 226 matching lines @@
     // investigation is needed to confirm that all cases would be handled
     // correctly (for example Link: preloads).
     if (committed_load_ &&
         creation_time >= committed_load_->navigation_start()) {
       return committed_load_.get();
     }
   }
   return nullptr;
 }
 
+void MetricsWebContentsObserver::OnRequestStarted(
+    const content::GlobalRequestID& request_id,
+    content::ResourceType resource_type,
+    base::TimeTicks creation_time) {
+  PageLoadTracker* tracker =
+      GetTrackerOrNullForRequest(request_id, resource_type, creation_time);
+  if (tracker) {
+    ExtraRequestInfo extra_request_info(

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

Debatable whether the two request methods should share a struct or each have
their own. Perhaps ExtraRequestStartInfo and ExtraRequestCompleteInfo. I'm not
sure how much this information will overlap because it seems like only
content::Type would be share right now.

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

OK, I'll separate it into two types (and add the request_type to the both)

+        false /*was_cached*/, 0 /*raw_body_bytes*/,
+        false /*used_data_reduction_proxy*/, 0 /*original_content_length*/);
+    tracker->OnStartedResource(extra_request_info);
+  }
+}
+
 void MetricsWebContentsObserver::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) {
   PageLoadTracker* tracker =
       GetTrackerOrNullForRequest(request_id, resource_type, creation_time);
@@ skipping 316 matching lines @@
     content::NavigationHandle* navigation_handle) const {
   DCHECK(navigation_handle->IsInMainFrame());
   DCHECK(!navigation_handle->HasCommitted() ||
          !navigation_handle->IsSameDocument());
 
   return BrowserPageTrackDecider(embedder_interface_.get(), web_contents(),
                                  navigation_handle).ShouldTrack();
 }
 
 }  // namespace page_load_metrics