Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/media/midi_dispatcher.cc

Index: content/renderer/media/midi_dispatcher.cc
diff --git a/content/renderer/media/midi_dispatcher.cc b/content/renderer/media/midi_dispatcher.cc
index 65f42a95d442c749ef269c0a703b46607bc5a297..49fe875537c21b2475b4e3c930075ee7f6ae20f9 100644
--- a/content/renderer/media/midi_dispatcher.cc
+++ b/content/renderer/media/midi_dispatcher.cc
@@ -24,6 +24,10 @@ MidiDispatcher::MidiDispatcher(RenderFrame* render_frame)
 
 MidiDispatcher::~MidiDispatcher() {}
 
+void MidiDispatcher::OnDestruct() {
+  delete this;
+}
+
 void MidiDispatcher::requestPermission(const WebMIDIPermissionRequest& request,
                                        const WebMIDIOptions& options) {
   if (!permission_service_.get()) {