Determine the layoutobject of middleClickAutoscroll by the scroll direction.

third_party/WebKit/Source/core/layout/LayoutBox.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 980 matching lines @@
                                               : verticalScrollbarWidth();
   }
   virtual ScrollResult scroll(ScrollGranularity, const FloatSize&);
   bool canBeScrolledAndHasScrollableArea() const;
   virtual bool canBeProgramaticallyScrolled() const;
   virtual void autoscroll(const IntPoint&);
   bool canAutoscroll() const;
   IntSize calculateAutoscrollDirection(const IntPoint& pointInRootFrame) const;
   static LayoutBox* findAutoscrollable(LayoutObject*);
   virtual void stopAutoscroll() {}
-  virtual void middleClickAutoscroll(const IntPoint&);
 
   DISABLE_CFI_PERF bool hasAutoVerticalScrollbar() const {
     return hasOverflowClip() && (style()->overflowY() == OverflowAuto ||
                                  style()->overflowY() == OverflowPagedY ||
                                  style()->overflowY() == OverflowOverlay);
   }
   DISABLE_CFI_PERF bool hasAutoHorizontalScrollbar() const {
     return hasOverflowClip() && (style()->overflowX() == OverflowAuto ||
                                  style()->overflowX() == OverflowOverlay);
   }
@@ skipping 609 matching lines @@
 
 inline bool LayoutBox::isForcedFragmentainerBreakValue(EBreak breakValue) {
   return breakValue == BreakColumn || breakValue == BreakLeft ||
          breakValue == BreakPage || breakValue == BreakRecto ||
          breakValue == BreakRight || breakValue == BreakVerso;
 }
 
 }  // namespace blink
 
 #endif  // LayoutBox_h