Add virtual destructors for the Pepper API thunks.

ppapi/thunk/ppb_directory_reader_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_DIRECTORY_READER_API_H_
 #define PPAPI_THUNK_DIRECTORY_READER_API_H_
 
 #include "ppapi/c/dev/ppb_directory_reader_dev.h"
 
 namespace ppapi {
 namespace thunk {
 
 class PPB_DirectoryReader_API {
  public:
+  virtual ~PPB_DirectoryReader_API() {}
+
   virtual int32_t GetNextEntry(PP_DirectoryEntry_Dev* entry,
                                PP_CompletionCallback callback) = 0;
 };
 
 }  // namespace thunk
 }  // namespace ppapi
 
 #endif  // PPAPI_THUNK_DIRECTORY_READER_API_H_