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

chrome/browser/page_load_metrics/observers/resource_tracking_page_load_metrics_observer.h

+// 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.
+
+#ifndef CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_RESOURCE_TRACKING_PAGE_LOAD_METRICS_OBSERVER_H_
+#define CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_RESOURCE_TRACKING_PAGE_LOAD_METRICS_OBSERVER_H_
+
+#include <stdint.h>
+
+#include "chrome/browser/page_load_metrics/page_load_metrics_observer.h"
+
+namespace page_load_metrics {
+
+class ResourceTrackingPageLoadMetricsObserver
+    : public page_load_metrics::PageLoadMetricsObserver {
+ public:
+  ResourceTrackingPageLoadMetricsObserver();
+  ~ResourceTrackingPageLoadMetricsObserver() override;
+
+  // Called by the PageLoadMetrics framework when we start a new request, so we
+  // can update our data structures to be able to calculate a resource done
+  // percentage.
+  void OnStartedResource(
+      const ExtraRequestStartInfo& extra_request_start_info) override;
+
+  // Called by the PageLoadMetrics framework when we start a new request, so we
+  // can update our data structures to be able to calculate a resource done
+  // percentage.
+  void OnLoadedResource(
+      const ExtraRequestCompleteInfo& extra_request_complete_info) override;
+
+  // For the specified type, get the count of requests started and completed.

[RyanSturm, 2017/04/17 19:57:08]

Can you update this comment to reflect that |type| is currently unused until it
is used?

[Pete Williamson, 2017/04/17 20:46:36]

Done.

+  void GetCountsForType(const content::ResourceType type,
+                        int64_t* started_count,
+                        int64_t* completed_count);
+
+ private:
+  // TODO(petewil): Some way to keep track of what we've seen
+  // TODO(petewil): Some way to inform our keeper of aggregate results when they
+  // change.};

[RyanSturm, 2017/04/17 19:57:08]

nit: s/change.};/change./

[Pete Williamson, 2017/04/17 20:46:36]

Done.

+  int64_t started_count_;
+  int64_t completed_count_;
+};
+
+}  // namespace page_load_metrics
+
+#endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_RESOURCE_TRACKING_PAGE_LOAD_METRICS_OBSERVER_H_