Don't call into ScrollView from Scrollbar.

Source/platform/scroll/ScrollableArea.h

 /*
  * Copyright (C) 2008, 2011 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 140 matching lines @@
     {
         return scrollbar->Widget::convertToContainingView(scrollbarPoint);
     }
     virtual IntPoint convertFromContainingViewToScrollbar(const Scrollbar* scrollbar, const IntPoint& parentPoint) const
     {
         return scrollbar->Widget::convertFromContainingView(parentPoint);
     }
 
     virtual Scrollbar* horizontalScrollbar() const { return 0; }
     virtual Scrollbar* verticalScrollbar() const { return 0; }
+    virtual bool isScrollViewScrollbar(const Widget* child) const { return false; }
 
     // scrollPosition is relative to the scrollOrigin. i.e. If the page is RTL
     // then scrollPosition will be negative. By default, scrollPositionDouble()
     // just call into scrollPosition(). Subclass can override scrollPositionDouble()
     // to return floating point precision scrolloffset.
     // FIXME: Remove scrollPosition(). crbug.com/414283.
     virtual IntPoint scrollPosition() const = 0;
     virtual DoublePoint scrollPositionDouble() const { return DoublePoint(scrollPosition()); }
     virtual IntPoint minimumScrollPosition() const = 0;
     virtual IntPoint maximumScrollPosition() const = 0;
@@ skipping 136 matching lines @@
     // vertical-lr / ltr            NO                      NO
     // vertical-lr / rtl            NO                      YES
     // vertical-rl / ltr            YES                     NO
     // vertical-rl / rtl            YES                     YES
     IntPoint m_scrollOrigin;
 };
 
 } // namespace blink
 
 #endif // ScrollableArea_h