Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

extensions/renderer/render_frame_observer_natives.cc

Index: extensions/renderer/render_frame_observer_natives.cc
diff --git a/extensions/renderer/render_frame_observer_natives.cc b/extensions/renderer/render_frame_observer_natives.cc
index bd5d2377fadb17027bd2ec10045b185f53e05a57..1d22c5795ade695df1e2b2f92936aa2d717f5e60 100644
--- a/extensions/renderer/render_frame_observer_natives.cc
+++ b/extensions/renderer/render_frame_observer_natives.cc
@@ -39,6 +39,8 @@ class LoadWatcher : public content::RenderFrameObserver {
     delete this;
   }
 
+  void OnDestruct() override { delete this; }
+
  private:
   base::Callback<void(bool)> callback_;