Call ScrollableArea::ShowOverlayScrollbars for explicit scrolls only.

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 76 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 old_position,
       float new_position) const {
     return kAllParts;
   }
 
-  // Returns parts of the scrollbar which must be repainted following a change
-  // in enabled state.
-  virtual ScrollbarPart InvalidateOnEnabledChange() const { return kAllParts; }
-
   virtual void PaintScrollCorner(GraphicsContext&,
                                  const DisplayItemClient&,
                                  const IntRect& corner_rect);
   virtual void PaintTickmarks(GraphicsContext&,
                               const Scrollbar&,
                               const IntRect&);
 
   virtual bool ShouldCenterOnThumb(const ScrollbarThemeClient&,
                                    const WebMouseEvent&);
   virtual bool ShouldSnapBackToDragOrigin(const ScrollbarThemeClient&,
@@ skipping 103 matching lines @@
   static DisplayItem::Type TrackPiecePartToDisplayItemType(ScrollbarPart);
 
  private:
   static ScrollbarTheme&
   NativeTheme();  // Must be implemented to return the correct theme subclass.
   static bool g_mock_scrollbars_enabled_;
 };
 
 }  // namespace blink
 #endif