Aura overlay scrollbars shouldn't invalidate thumb on scroll or enabled state.

third_party/WebKit/Source/platform/scroll/ScrollbarTheme.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 74 matching lines @@
 
   // Returns parts of the scrollbar which must be repainted following a change
   // in the thumb position, given scroll positions before and after.
   virtual ScrollbarPart invalidateOnThumbPositionChange(
       const ScrollbarThemeClient&,
       float oldPosition,
       float newPosition) const {
     return AllParts;
   }
 
+  // Returns parts of the scrollbar which must be repainted following a change
+  // in enabled state.
+  virtual ScrollbarPart invalidateOnEnabledChange() const { return AllParts; }
+
   virtual void paintScrollCorner(GraphicsContext&,
                                  const DisplayItemClient&,
                                  const IntRect& cornerRect);
   virtual void paintTickmarks(GraphicsContext&,
                               const Scrollbar&,
                               const IntRect&);
 
   virtual bool shouldCenterOnThumb(const ScrollbarThemeClient&,
                                    const PlatformMouseEvent&);
   virtual bool shouldSnapBackToDragOrigin(const ScrollbarThemeClient&,
@@ skipping 85 matching lines @@
   static DisplayItem::Type trackPiecePartToDisplayItemType(ScrollbarPart);
 
  private:
   static ScrollbarTheme&
   nativeTheme();  // Must be implemented to return the correct theme subclass.
   static bool gMockScrollbarsEnabled;
 };
 
 }  // namespace blink
 #endif