Disable overlay scrollbars to hide them on non-Mac.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 472 matching lines @@
   bool hasHorizontalOverflow() const;
   bool hasVerticalOverflow() const;
   bool hasScrollableHorizontalOverflow() const;
   bool hasScrollableVerticalOverflow() const;
   bool visualViewportSuppliesScrollbars() const;
 
   bool needsScrollbarReconstruction() const;
 
   void updateScrollOrigin();
   void updateScrollDimensions();
+  void updateScrollbarEnabledState();
 
   void updateScrollOffset(const ScrollOffset&, ScrollType) override;
 
   int verticalScrollbarStart(int minX, int maxX) const;
   int horizontalScrollbarStart(int minX) const;
   IntSize scrollbarOffset(const Scrollbar&) const;
 
   enum ComputeScrollbarExistenceOption { Default, ForbidAddingAutoBars };
   void computeScrollbarExistence(
       bool& needsHorizontalScrollbar,
@@ skipping 79 matching lines @@
 
 DEFINE_TYPE_CASTS(PaintLayerScrollableArea,
                   ScrollableArea,
                   scrollableArea,
                   scrollableArea->isPaintLayerScrollableArea(),
                   scrollableArea.isPaintLayerScrollableArea());
 
 }  // namespace blink
 
 #endif  // LayerScrollableArea_h