Hide non-composited native scrollbars.

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 19 matching lines @@
 
 #ifndef ScrollbarThemeOverlayMock_h
 #define ScrollbarThemeOverlayMock_h
 
 #include "platform/scroll/ScrollbarThemeOverlay.h"
 
 namespace blink {
 
 class PLATFORM_EXPORT ScrollbarThemeOverlayMock : public ScrollbarThemeOverlay {
 public:
-    ScrollbarThemeOverlayMock() : ScrollbarThemeOverlay(3, 4, DisallowHitTest, Color(128, 128, 128)) { }
+    enum VisibilityOverride { DefaultVisibility, HideScrollbars };
+
+    ScrollbarThemeOverlayMock(VisibilityOverride visibility = DefaultVisibility)
+        : ScrollbarThemeOverlay(
+            (visibility == HideScrollbars) ? 0 : 3,
+            (visibility == HideScrollbars) ? 0 : 4,
+            DisallowHitTest,
+            (visibility == HideScrollbars) ? Color() : Color(128, 128, 128)) { }
 
 private:
     bool isMockTheme() const final { return true; }
 };
 
 } // namespace blink
 #endif // ScrollbarThemeOverlayMock_h