Fix chromium-style warnings in webrtc/sound/.

webrtc/sound/nullsoundsystemfactory.h

 /*
  *  Copyright 2004 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #ifndef WEBRTC_SOUND_NULLSOUNDSYSTEMFACTORY_H_
 #define WEBRTC_SOUND_NULLSOUNDSYSTEMFACTORY_H_
 
 #include "webrtc/sound/soundsystemfactory.h"
 
 namespace rtc {
 
 // A SoundSystemFactory that always returns a NullSoundSystem. Intended for
 // testing.
 class NullSoundSystemFactory : public SoundSystemFactory {
  public:
   NullSoundSystemFactory();
-  virtual ~NullSoundSystemFactory();
+  ~NullSoundSystemFactory() override;
 
  protected:
   // Inherited from SoundSystemFactory.
-  virtual bool SetupInstance();
-  virtual void CleanupInstance();
+  bool SetupInstance() override;
+  void CleanupInstance() override;
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_SOUND_NULLSOUNDSYSTEMFACTORY_H_