Disable overlay scrollbars in Blink when hidden by the compositor.

third_party/WebKit/Source/core/frame/FrameView.h

 /*
    Copyright (C) 1997 Martin Jones (mjones@kde.org)
              (C) 1998 Waldo Bastian (bastian@kde.org)
              (C) 1998, 1999 Torben Weis (weis@kde.org)
              (C) 1999 Lars Knoll (knoll@kde.org)
              (C) 1999 Antti Koivisto (koivisto@kde.org)
    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights
    reserved.
 
    This library is free software; you can redistribute it and/or
@@ skipping 745 matching lines @@
       bool& newHasVerticalScrollbar,
       const IntSize& docSize,
       ComputeScrollbarExistenceOption = FirstPass) const;
   void updateScrollbarGeometry();
 
   // Called to update the scrollbars to accurately reflect the state of the
   // view.
   void updateScrollbars();
   void updateScrollbarsIfNeeded();
 
+  void didChangeScrollbarsHidden() override;
+
   class InUpdateScrollbarsScope {
     STACK_ALLOCATED();
 
    public:
     explicit InUpdateScrollbarsScope(FrameView* view)
         : m_scope(&view->m_inUpdateScrollbars, true) {}
 
    private:
     AutoReset<bool> m_scope;
   };
@@ skipping 337 matching lines @@
                   widget.isFrameView());
 DEFINE_TYPE_CASTS(FrameView,
                   ScrollableArea,
                   scrollableArea,
                   scrollableArea->isFrameView(),
                   scrollableArea.isFrameView());
 
 }  // namespace blink
 
 #endif  // FrameView_h