Disable overlay scrollbars to hide them on non-Mac.

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 796 matching lines @@
     // TODO(ymalik): This should be hidden and all calls should go through
     // setHas*Scrollbar functions above.
     Scrollbar* createScrollbar(ScrollbarOrientation) override;
 
    protected:
     void destroyScrollbar(ScrollbarOrientation) override;
   };
 
   void updateScrollOffset(const ScrollOffset&, ScrollType) override;
 
+  void updateScrollbarEnabledState();
+
   void updateLifecyclePhasesInternal(
       DocumentLifecycle::LifecycleState targetState);
 
   void invalidateTreeIfNeededRecursive();
   void scrollContentsIfNeededRecursive();
   void updateStyleAndLayoutIfNeededRecursive();
   void updatePaintProperties();
   void synchronizedPaint();
   void synchronizedPaintRecursively(GraphicsLayer*);
 
@@ skipping 322 matching lines @@
                   widget.isFrameView());
 DEFINE_TYPE_CASTS(FrameView,
                   ScrollableArea,
                   scrollableArea,
                   scrollableArea->isFrameView(),
                   scrollableArea.isFrameView());
 
 }  // namespace blink
 
 #endif  // FrameView_h