Update scrollbar layer opacity when scrollbar theme changes

Source/platform/scroll/ScrollView.h

 /*
  * Copyright (C) 2004, 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Holger Hans Peter Freyther
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 288 matching lines @@
     public:
         explicit InUpdateScrollbarsScope(ScrollView* view)
             : m_scope(view->m_inUpdateScrollbars, true)
         { }
     private:
         TemporaryChange<bool> m_scope;
     };
 
 private:
     bool adjustScrollbarExistence(ComputeScrollbarExistenceOption = FirstPass);
+    void adjustScrollbarOpacity();
 
     RefPtr<Scrollbar> m_horizontalScrollbar;
     RefPtr<Scrollbar> m_verticalScrollbar;
     ScrollbarMode m_horizontalScrollbarMode;
     ScrollbarMode m_verticalScrollbarMode;
 
     bool m_horizontalScrollbarLock;
     bool m_verticalScrollbarLock;
 
     HashSet<RefPtr<Widget> > m_children;
@@ skipping 21 matching lines @@
 
     void calculateOverhangAreasForPainting(IntRect& horizontalOverhangRect, IntRect& verticalOverhangRect);
     void updateOverhangAreas();
 }; // class ScrollView
 
 DEFINE_TYPE_CASTS(ScrollView, Widget, widget, widget->isScrollView(), widget.isScrollView());
 
 } // namespace WebCore
 
 #endif // ScrollView_h