Add virtual destructors for the Pepper API thunks.

ppapi/thunk/ppb_scrollbar_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_SCROLLBAR_API_H_
 #define PPAPI_THUNK_PPB_SCROLLBAR_API_H_
 
 #include "ppapi/c/dev/ppb_scrollbar_dev.h"
 
 namespace ppapi {
 namespace thunk {
 
 class PPB_Scrollbar_API {
  public:
+  virtual ~PPB_Scrollbar_API() {}
+
   virtual uint32_t GetThickness() = 0;
   virtual uint32_t GetValue() = 0;
   virtual void SetValue(uint32_t value) = 0;
   virtual void SetDocumentSize(uint32_t size) = 0;
   virtual void SetTickMarks(const PP_Rect* tick_marks, uint32_t count) = 0;
   virtual void ScrollBy(PP_ScrollBy_Dev unit, int32_t multiplier) = 0;
 };
 
 }  // namespace thunk
 }  // namespace ppapi
 
 #endif  // PPAPI_THUNK_PPB_SCROLLBAR_API_H_