Warn on missing OVERRIDE/virtual everywhere, not just in header files.

chrome/browser/extensions/api/audio/audio_service.cc

 // Copyright (c) 2013 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 "chrome/browser/extensions/api/audio/audio_service.h"
 
 namespace extensions {
 
 class AudioServiceImpl : public AudioService {
  public:
   AudioServiceImpl() {}
   virtual ~AudioServiceImpl() {}
 
   // Called by listeners to this service to add/remove themselves as observers.
-  virtual void AddObserver(Observer* observer);
-  virtual void RemoveObserver(Observer* observer);
+  virtual void AddObserver(Observer* observer) OVERRIDE;
+  virtual void RemoveObserver(Observer* observer) OVERRIDE;
 
   // Start to query audio device information.
-  virtual void StartGetInfo(const GetInfoCallback& callback);
+  virtual void StartGetInfo(const GetInfoCallback& callback) OVERRIDE;
 };
 
 void AudioServiceImpl::AddObserver(Observer* observer) {
   // TODO: implement this for platforms other than Chrome OS.
 }
 
 void AudioServiceImpl::RemoveObserver(Observer* observer) {
   // TODO: implement this for platforms other than Chrome OS.
 }
 
 AudioService* AudioService::CreateInstance() {
   return new AudioServiceImpl;
 }
 
 void AudioServiceImpl::StartGetInfo(const GetInfoCallback& callback) {
   // TODO: implement this for platforms other than Chrome OS.
   if (!callback.is_null())
     callback.Run(OutputInfo(), InputInfo(), false);
 }
 
 }  // namespace extensions