Oilpan: fix build after r369477.

third_party/WebKit/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 77 matching lines @@
 }
 
 Scrollbar::~Scrollbar()
 {
     m_theme.unregisterScrollbar(*this);
 }
 
 DEFINE_TRACE(Scrollbar)
 {
     visitor->trace(m_scrollableArea);
+    visitor->trace(m_hostWindow);
     Widget::trace(visitor);
 }
 
 void Scrollbar::setFrameRect(const IntRect& frameRect)
 {
     if (frameRect == this->frameRect())
         return;
 
     Widget::setFrameRect(frameRect);
     setNeedsPaintInvalidation();
@@ skipping 458 matching lines @@
         invalidParts = AllParts;
     if (invalidParts & ~ThumbPart)
         m_trackNeedsRepaint = true;
     if (invalidParts & ThumbPart)
         m_thumbNeedsRepaint = true;
     if (m_scrollableArea)
         m_scrollableArea->setScrollbarNeedsPaintInvalidation(orientation());
 }
 
 } // namespace blink