RLSA needsLayout on customscrollbar thickness change.

Source/core/rendering/RenderScrollbar.cpp

 /*
  * Copyright (C) 2008, 2009 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 186 matching lines @@
     int oldThickness = isHorizontal ? height() : width();
     int newThickness = 0;
     RenderScrollbarPart* part = m_parts.get(ScrollbarBGPart);
     if (part) {
         part->layout();
         newThickness = isHorizontal ? part->height() : part->width();
     }
 
     if (newThickness != oldThickness) {
         setFrameRect(IntRect(location(), IntSize(isHorizontal ? width() : newThickness, isHorizontal ? newThickness : height())));
-        if (RenderBox* box = owningRenderer())
+        if (RenderBox* box = owningRenderer()) {
+            if (box->isRenderBlock())
+                toRenderBlock(box)->notifyScrollbarThicknessChanged();
             box->setChildNeedsLayout();
+        }
     }
 }
 
 static PseudoId pseudoForScrollbarPart(ScrollbarPart part)
 {
     switch (part) {
         case BackButtonStartPart:
         case ForwardButtonStartPart:
         case BackButtonEndPart:
         case ForwardButtonEndPart:
@@ skipping 139 matching lines @@
 int RenderScrollbar::minimumThumbLength()
 {
     RenderScrollbarPart* partRenderer = m_parts.get(ThumbPart);
     if (!partRenderer)
         return 0;
     partRenderer->layout();
     return orientation() == HorizontalScrollbar ? partRenderer->width() : partRenderer->height();
 }
 
 }