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

chrome/browser/page_load_metrics/observers/resource_tracking_page_load_metrics_observer.cc

+// Copyright 2017 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/observers/resource_tracking_page_load_metrics_observer.h"
+
+namespace page_load_metrics {
+
+ResourceTrackingPageLoadMetricsObserver::
+    ResourceTrackingPageLoadMetricsObserver()
+    : started_count_(0), completed_count_(0) {}
+ResourceTrackingPageLoadMetricsObserver::
+    ~ResourceTrackingPageLoadMetricsObserver() {}
+
+void ResourceTrackingPageLoadMetricsObserver::OnStartedResource(
+    const ExtraRequestStartInfo& extra_request_start_info) {
+  // TODO(petewiL): Store this by type.
+  ++started_count_;
+}
+
+void ResourceTrackingPageLoadMetricsObserver::OnLoadedResource(
+    const ExtraRequestCompleteInfo& extra_request_complete_info) {
+  // TODO(petewil): Check to see if the type of the request changed.  If it did,
+  // update the old and new types for the started type.  Then update by type for
+  // the completed type.
+  ++completed_count_;
+}
+
+void ResourceTrackingPageLoadMetricsObserver::GetCountsForType(
+    const content::ResourceType type,
+    int64_t& started_count,

[RyanSturm, 2017/04/14 20:08:43]

I think there is a style point against non-const reference variables. I'd
personally prefer std::pair as the return type, but passing in pointers instead
of refs is the other option.

[Pete Williamson, 2017/04/17 18:53:11]

Done (Passing pointers instead of references).

+    int64_t& completed_count) {
+  started_count = started_count_;
+  completed_count = completed_count_;
+}
+
+}  // namespace page_load_metrics