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

[Bryan McQuade, 2017/04/17 23:32:13]

is your plan for this class to extend it to also provide data to your class? if
so i think adding this here is ok. if this is only being added in order to write
unit tests, though, i think i'm inclined to instead move this class to a
unittest-only file to test this behavior.

if your plan is to use this class, do you have a follow up change that shows how
this class will be used? i'd be interested to see what that's going to look like
as well.

[Pete Williamson, 2017/04/18 18:17:27]

Design doc is here:
https://docs.google.com/document/d/1dxGOMsUkmxfoNUfQKqZRi4qAhWVGh9DZDiFI1arkiJ0
Section titled Hookup the PLMO from offline pages code explains the plan in more
detail.
Basically, the plan is for this observer to forward the data to our Offliner
class.  I haven't coded that
changelist yet, but I will send the change to you for review when it is ready.

+    : 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.
+  // TODO(petewil) Note that type is not used yet, code to use it is coming
+  // soon.
+  void GetCountsForType(const content::ResourceType type,

[Bryan McQuade, 2017/04/17 23:32:14]

we've avoided having getters from any of the observers, for any purpose other
than testing. if this is just for testing purpose then let's call it
GetCountsForTypeForTesting() to make that clear. If the intent is to call this
from other places in chrome, I'd like to see if we can instead have the observer
notify the interested consumer of the updated info. This is what we do in the
android observer for example. The reason to avoid getting the observer is that
its lifetime is bound to a page load & so I generally worry about callers having
a pointer to the object, since it can go away at any time.

[Pete Williamson, 2017/04/18 18:17:27]

My original plan was to have the getter available to clients, but everything I
need can be 
accomplished by having the observer pass along notifications to the Offliner, so
I'll change
this into a 'for testing' method.

+                        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.
+  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_