Add virtual destructors for the Pepper API thunks.

ppapi/thunk/ppb_audio_config_api.h

 // Copyright (c) 2011 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 PPAPI_THUNK_AUDIO_CONFIG_API_H_
 #define PPAPI_THUNK_AUDIO_CONFIG_API_H_
 
 #include "ppapi/c/ppb_audio_config.h"
 
 namespace ppapi {
 namespace thunk {
 
 class PPB_AudioConfig_API {
  public:
+  virtual ~PPB_AudioConfig_API() {}
+
   virtual PP_AudioSampleRate GetSampleRate() = 0;
   virtual uint32_t GetSampleFrameCount() = 0;
 };
 
 }  // namespace thunk
 }  // namespace ppapi
 
 #endif  // PPAPI_THUNK_AUDIO_CONFIG_API_H_