Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/stats_collection_observer.h

Index: content/renderer/stats_collection_observer.h
diff --git a/content/renderer/stats_collection_observer.h b/content/renderer/stats_collection_observer.h
index 5a17dfe6933bed07d518e4a6ac4fee84e414ee20..e1b73233120df2ffa2a45b70c195a3e10d814649 100644
--- a/content/renderer/stats_collection_observer.h
+++ b/content/renderer/stats_collection_observer.h
@@ -30,6 +30,9 @@ class StatsCollectionObserver : public RenderViewObserver {
   const base::Time& load_stop_time() { return stop_time_; }
 
  private:
+  // RenderViewObserver implementation.
+  void OnDestruct() override;
+
   base::Time start_time_;
   base::Time stop_time_;