Add virtual destructors for the Pepper API thunks.

ppapi/thunk/ppb_image_data_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_PPB_IMAGE_DATA_API_H_
 #define PPAPI_THUNK_PPB_IMAGE_DATA_API_H_
 
 #include "ppapi/c/pp_bool.h"
 #include "ppapi/c/ppb_image_data.h"
 
 namespace ppapi {
 namespace thunk {
 
 class PPB_ImageData_API {
  public:
+  virtual ~PPB_ImageData_API() {}
+
   virtual PP_Bool Describe(PP_ImageDataDesc* desc) = 0;
   virtual void* Map() = 0;
   virtual void Unmap() = 0;
 
   // Trusted inteface.
   virtual int32_t GetSharedMemory(int* handle, uint32_t* byte_count) = 0;
 };
 
 }  // namespace thunk
 }  // namespace ppapi
 
 #endif  // PPAPI_THUNK_PPB_IMAGE_DATA_API_H_