Determine the layoutobject of middleClickAutoscroll by the scroll direction.

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

Index: third_party/WebKit/Source/core/page/AutoscrollController.cpp
diff --git a/third_party/WebKit/Source/core/page/AutoscrollController.cpp b/third_party/WebKit/Source/core/page/AutoscrollController.cpp
index 454f53380c05baf2936ed47c1828d3c95a4e3c46..764ba35505d04e1584f5d863906576fc9c14490b 100644
--- a/third_party/WebKit/Source/core/page/AutoscrollController.cpp
+++ b/third_party/WebKit/Source/core/page/AutoscrollController.cpp
@@ -54,7 +54,8 @@ AutoscrollController::AutoscrollController(Page& page)
       m_autoscrollLayoutObject(nullptr),
       m_pressedLayoutObject(nullptr),
       m_autoscrollType(NoAutoscroll),
-      m_dragAndDropAutoscrollStartTime(0) {}
+      m_dragAndDropAutoscrollStartTime(0),
+      m_middleClickAutoscrolled(false) {}
 
 DEFINE_TRACE(AutoscrollController) {
   visitor->trace(m_page);
@@ -120,6 +121,15 @@ void AutoscrollController::stopAutoscrollIfNeeded(LayoutObject* layoutObject) {
   m_autoscrollType = NoAutoscroll;
 }
 
+bool AutoscrollController::setMiddleClickAutoscrollLayoutObject(
+    LayoutBox* layoutObject) {
+  if (!m_middleClickAutoscrolled && layoutObject) {
+    m_autoscrollLayoutObject = layoutObject;
+    return true;
+  }
+  return false;
+}
+
 void AutoscrollController::updateAutoscrollLayoutObject() {
   if (!m_autoscrollLayoutObject)
     return;
@@ -234,6 +244,7 @@ void AutoscrollController::startMiddleClickAutoscroll(
   m_autoscrollType = AutoscrollForMiddleClick;
   m_autoscrollLayoutObject = scrollable;
   m_middleClickAutoscrollStartPos = lastKnownMousePosition;
+  m_middleClickAutoscrolled = false;
 
   UseCounter::count(m_page->mainFrame(),
                     UseCounter::MiddleClickAutoscrollStart);