Add virtual destructors for the Pepper API thunks.

ppapi/thunk/ppb_widget_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_WIDGET_API_H_
 #define PPAPI_THUNK_PPB_WIDGET_API_H_
 
 #include "ppapi/c/dev/ppb_widget_dev.h"
 
 namespace ppapi {
 namespace thunk {
 
 class PPB_Widget_API {
  public:
+  virtual ~PPB_Widget_API() {}
+
   virtual PP_Bool Paint(const PP_Rect* rect, PP_Resource image_id) = 0;
   virtual PP_Bool HandleEvent(const PP_InputEvent* event) = 0;
   virtual PP_Bool GetLocation(PP_Rect* location) = 0;
   virtual void SetLocation(const PP_Rect* location) = 0;
 };
 
 }  // namespace thunk
 }  // namespace ppapi
 
 #endif  // PPAPI_THUNK_PPB_WIDGET_API_H_