Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/accessibility/renderer_accessibility.cc

 // Copyright (c) 2012 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.
 
 #include "content/renderer/accessibility/renderer_accessibility.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <queue>
@@ skipping 590 matching lines @@
   if (obj.isDetached()) {
 #ifndef NDEBUG
     LOG(WARNING) << "ShowContextMenu on invalid object id " << acc_obj_id;
 #endif
     return;
   }
 
   obj.showContextMenu();
 }
 
+void RendererAccessibility::OnDestruct() {
+  delete this;
+}
+
 }  // namespace content