Determine the layoutobject of middleClickAutoscroll by the scroll direction.

third_party/WebKit/Source/core/page/AutoscrollController.h

Index: third_party/WebKit/Source/core/page/AutoscrollController.h
diff --git a/third_party/WebKit/Source/core/page/AutoscrollController.h b/third_party/WebKit/Source/core/page/AutoscrollController.h
index b20c42dd70a5afab63209846587d997cdcc26895..dfef498cad032821b9bdbcde880e251e003c990c 100644
--- a/third_party/WebKit/Source/core/page/AutoscrollController.h
+++ b/third_party/WebKit/Source/core/page/AutoscrollController.h
@@ -72,6 +72,10 @@ class CORE_EXPORT AutoscrollController final
   void handleMouseReleaseForMiddleClickAutoscroll(LocalFrame*,
                                                   const PlatformMouseEvent&);
   void startMiddleClickAutoscroll(LayoutBox*, const IntPoint&);
+  bool setMiddleClickAutoscrollLayoutObject(LayoutBox*);
+  void setMiddleClickAutoscrolled(bool scrolled) {
+    m_middleClickAutoscrolled = scrolled;
+  };
 
  private:
   explicit AutoscrollController(Page&);
@@ -88,6 +92,7 @@ class CORE_EXPORT AutoscrollController final
   IntPoint m_dragAndDropAutoscrollReferencePosition;
   double m_dragAndDropAutoscrollStartTime;
   IntPoint m_middleClickAutoscrollStartPos;
+  bool m_middleClickAutoscrolled;
 };
 
 }  // namespace blink