RLSA needsLayout on customscrollbar thickness change.

Source/core/rendering/RenderBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 77 matching lines @@
     return documentElementRenderer
         && !documentElementRenderer->hasBackground()
         && (documentElementRenderer == bodyElementRenderer->parent());
 }
 
 RenderBox::RenderBox(ContainerNode* node)
     : RenderBoxModelObject(node)
     , m_intrinsicContentLogicalHeight(-1)
     , m_minPreferredLogicalWidth(-1)
     , m_maxPreferredLogicalWidth(-1)
+    , m_customScrollbarThicknessChanged(false)
 {
     setIsBox();
 }
 
 void RenderBox::willBeDestroyed()
 {
     clearOverrideSize();
     clearContainingBlockOverrideSize();
 
     RenderBlock::removePercentHeightDescendantIfNeeded(this);
@@ skipping 4331 matching lines @@
 
     setLogicalTop(oldLogicalTop);
     setLogicalWidth(oldLogicalWidth);
     setMarginLeft(oldMarginLeft);
     setMarginRight(oldMarginRight);
 
     return borderBox;
 }
 
 } // namespace blink