Disable overlay scrollbars to hide them on non-Mac.

third_party/WebKit/Source/platform/scroll/ScrollbarThemeMac.h

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  *
  * 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 33 matching lines @@
   void registerScrollbar(ScrollbarThemeClient&) override;
   void unregisterScrollbar(ScrollbarThemeClient&) override;
   void preferencesChanged(float initialButtonDelay,
                           float autoscrollButtonDelay,
                           NSScrollerStyle preferredScrollerStyle,
                           bool redraw,
                           WebScrollbarButtonsPlacement);
 
   bool supportsControlTints() const override { return true; }
 
+  // On Mac, the painting code itself animates the opacity so there's no need
+  // to disable in order to make the scrollbars invisible. In fact,
+  // disabling/enabling causes invalidations which can cause endless loops as
+  // Mac queues up scrollbar paint timers.
+  bool shouldDisableInvisibleScrollbars() const override { return false; }
+
   double initialAutoscrollTimerDelay() override;
   double autoscrollTimerDelay() override;
 
   void paintTickmarks(GraphicsContext&,
                       const Scrollbar&,
                       const IntRect&) override;
 
   bool shouldRepaintAllPartsOnInvalidation() const override { return false; }
   ScrollbarPart invalidateOnThumbPositionChange(
       const ScrollbarThemeClient&,
@@ skipping 40 matching lines @@
 
   int minimumThumbLength(const ScrollbarThemeClient&) override;
 
   int tickmarkBorderWidth() override { return 1; }
 
   RefPtr<Pattern> m_overhangPattern;
 };
 }
 
 #endif  // ScrollbarThemeMac_h