[PageLoadMetrics] Handle case where navigation aborts

chrome/browser/page_load_metrics/observers/ads_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_ADS_PAGE_LOAD_METRICS_OBSERVER_H_
 #define CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_ADS_PAGE_LOAD_METRICS_OBSERVER_H_
 
 #include <list>
 #include <map>
 #include <memory>
 
 #include "base/macros.h"
 #include "chrome/browser/page_load_metrics/page_load_metrics_observer.h"
 #include "net/http/http_response_info.h"
 
+// This observer labels each sub-frame as an ad or not, and keeps track of
+// relevant per-frame and whole-page byte statistics.
 class AdsPageLoadMetricsObserver
     : public page_load_metrics::PageLoadMetricsObserver {
  public:
   // Returns a new AdsPageLoadMetricObserver. If the feature is disabled it
   // returns nullptr.
   static std::unique_ptr<AdsPageLoadMetricsObserver> CreateIfNeeded();
 
   AdsPageLoadMetricsObserver();
   ~AdsPageLoadMetricsObserver() override;
 
@@ skipping 47 matching lines @@
 
   size_t page_bytes_ = 0u;
   size_t uncached_page_bytes_ = 0u;
   int top_level_subframe_count_ = 0;
   int top_level_ad_frame_count_ = 0;
 
   DISALLOW_COPY_AND_ASSIGN(AdsPageLoadMetricsObserver);
 };
 
 #endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_OBSERVERS_ADS_PAGE_LOAD_METRICS_OBSERVER_H_