Fixed Inconsistent behaviour of custom-scrollbar

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
    modify it under the terms of the GNU Library General Public
@@ skipping 577 matching lines @@
     void performPostLayoutTasks();
 
     void invalidateTreeIfNeeded();
 
     void gatherDebugLayoutRects(RenderObject* layoutRoot);
 
     DocumentLifecycle& lifecycle() const;
 
     void contentRectangleForPaintInvalidation(const IntRect&);
     virtual void contentsResized() override;
-    void scrollbarExistenceDidChange();
+    void scrollbarNeedsLayout();
 
     // Override Widget methods to do point conversion via renderers, in order to
     // take transforms into account.
     virtual IntRect convertToContainingView(const IntRect&) const override;
     virtual IntRect convertFromContainingView(const IntRect&) const override;
     virtual IntPoint convertToContainingView(const IntPoint&) const override;
     virtual IntPoint convertFromContainingView(const IntPoint&) const override;
 
     void updateWidgetPositionsIfNeeded();
 
@@ skipping 218 matching lines @@
     }
 
 private:
     RawPtrWillBeMember<FrameView> m_view;
     bool m_originalValue;
 };
 
 } // namespace blink
 
 #endif // FrameView_h