Makes RenderFrameObserver/RenderViewObserver::OnDestruct pure virtual.

content/renderer/media/android/renderer_media_session_manager.h

 // Copyright 2015 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.
 
 #ifndef CONTENT_RENDERER_MEDIA_ANDROID_RENDERER_MEDIA_SESSION_MANAGER_H_
 #define CONTENT_RENDERER_MEDIA_ANDROID_RENDERER_MEDIA_SESSION_MANAGER_H_
 
 #include <map>
 #include <memory>
 
@@ skipping 26 matching lines @@
       int session_id,
       std::unique_ptr<blink::WebMediaSessionDeactivateCallback> callback);
   void SetMetadata(int session_id, const MediaMetadata& metadata);
 
   void OnDidActivate(int request_id, bool success);
   void OnDidDeactivate(int request_id);
 
  private:
   friend class WebMediaSessionTest;
 
+  // RenderFrameObserver implementation.
+  void OnDestruct() override;
+
   std::map<int, WebMediaSessionAndroid*> sessions_;
   int next_session_id_;
 
   using ActivationRequests =
       IDMap<blink::WebMediaSessionActivateCallback, IDMapOwnPointer>;
   ActivationRequests pending_activation_requests_;
 
   using DeactivationRequests =
       IDMap<blink::WebMediaSessionDeactivateCallback, IDMapOwnPointer>;
   DeactivationRequests pending_deactivation_requests_;
 
   DISALLOW_COPY_AND_ASSIGN(RendererMediaSessionManager);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_MEDIA_ANDROID_RENDERER_MEDIA_SESSION_MANAGER_H_