[subresource_filter] Use the SubresourceFilterObserver for PageLoadMetrics

components/subresource_filter/content/browser/subresource_filter_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 COMPONENTS_SUBRESOURCE_FILTER_CONTENT_BROWSER_SUBRESOURCE_FILTER_OBSERVER_H_
 #define COMPONENTS_SUBRESOURCE_FILTER_CONTENT_BROWSER_SUBRESOURCE_FILTER_OBSERVER_H_
 
 #include "components/subresource_filter/core/common/activation_decision.h"
 
 namespace content {
 class NavigationHandle;
 }  // namespace content
 
 namespace subresource_filter {
 
 struct ActivationState;
 
 // Class to receive notifications of subresource filter events for a given
 // WebContents. Registered with a SubresourceFilterObserverManager.
 class SubresourceFilterObserver {
  public:
   virtual ~SubresourceFilterObserver() = default;
 
+  // Called before the observer manager is destroyed. Observers must unregister
+  // themselves by this point.
   virtual void OnSubresourceFilterGoingAway() {}
 
+  // Called at most once per navigation when page activation is computed. This
+  // will be called before ReadyToCommitNavigation.
   virtual void OnPageActivationComputed(
       content::NavigationHandle* navigation_handle,
       ActivationDecision activation_decision,
       const ActivationState& activation_state) {}
 };
 
 }  // namespace subresource_filter
 
 #endif  // COMPONENTS_SUBRESOURCE_FILTER_CONTENT_BROWSER_SUBRESOURCE_FILTER_OBSERVER_H_