Oilpan: disconnected scrollbars must be externally observable as such.

Source/platform/scroll/Scrollbar.cpp

 /*
  * Copyright (C) 2004, 2006, 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 125 matching lines @@
 
     int oldThumbPosition = theme()->thumbPosition(this);
     m_currentPos = position;
     updateThumbPosition();
     if (m_pressedPart == ThumbPart)
         setPressedPos(m_pressedPos + theme()->thumbPosition(this) - oldThumbPosition);
 }
 
 void Scrollbar::disconnectFromScrollableArea()
 {
-#if !ENABLE(OILPAN)
-    // TODO(Oilpan): remove method once transitioned.
     m_scrollableArea = nullptr;
-#endif
 }
 
 void Scrollbar::setProportion(int visibleSize, int totalSize)
 {
     if (visibleSize == m_visibleSize && totalSize == m_totalSize)
         return;
 
     m_visibleSize = visibleSize;
     m_totalSize = totalSize;
 
@@ skipping 393 matching lines @@
     if (!m_scrollableArea)
         return 0;
 
     if (m_orientation == HorizontalScrollbar)
         return m_scrollableArea->scrollPosition().x() - m_scrollableArea->minimumScrollPosition().x();
 
     return m_scrollableArea->scrollPosition().y() - m_scrollableArea->minimumScrollPosition().y();
 }
 
 } // namespace blink