Disable overlay scrollbars to hide them on non-Mac.

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

 /*
  * Copyright (C) 2013 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 32 matching lines @@
 
   double overlayScrollbarFadeOutDelaySeconds() const override {
     return m_delayInSeconds;
   }
   double overlayScrollbarFadeOutDurationSeconds() const override { return 0.0; }
 
   void setOverlayScrollbarFadeOutDelay(double delayInSeconds) {
     m_delayInSeconds = delayInSeconds;
   }
 
+  void paintThumb(GraphicsContext& gc,
+                  const Scrollbar& scrollbar,
+                  const IntRect& rect) override {
+    if (!scrollbar.enabled())
+      return;
+    ScrollbarThemeOverlay::paintThumb(gc, scrollbar, rect);
+  }
+
  private:
   double m_delayInSeconds;
   bool isMockTheme() const final { return true; }
 };
 
 }  // namespace blink
 #endif  // ScrollbarThemeOverlayMock_h