Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/media/midi_dispatcher.h

Index: content/renderer/media/midi_dispatcher.h
diff --git a/content/renderer/media/midi_dispatcher.h b/content/renderer/media/midi_dispatcher.h
index 802328a770cd647ac2fea150aa62726a9888f50e..2d98117d092771a4742b5fcdaa2912c9fb7be455 100644
--- a/content/renderer/media/midi_dispatcher.h
+++ b/content/renderer/media/midi_dispatcher.h
@@ -28,6 +28,9 @@ class MidiDispatcher : public RenderFrameObserver,
   ~MidiDispatcher() override;
 
  private:
+  // RenderFrameObserver implementation.
+  void OnDestruct() override;
+
   // blink::WebMIDIClient implementation.
   void requestPermission(const blink::WebMIDIPermissionRequest& request,
                          const blink::WebMIDIOptions& options) override;