RLSA needsLayout on customscrollbar thickness change.

Source/core/rendering/RenderLayerScrollableArea.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 220 matching lines @@
     unsigned m_scrollDimensionsDirty : 1;
     unsigned m_inOverflowRelayout : 1;
 
     RenderLayer* m_nextTopmostScrollChild;
     RenderLayer* m_topmostScrollChild;
 
     // FIXME: once cc can handle composited scrolling with clip paths, we will
     // no longer need this bit.
     unsigned m_needsCompositedScrolling : 1;
 
+    unsigned m_horizontalScrollbarRectChanged : 1;
+    unsigned m_verticalScrollbarRectChanged : 1;
+
     // The width/height of our scrolled area.
     LayoutRect m_overflowRect;
 
     // This is the (scroll) offset from scrollOrigin().
     DoubleSize m_scrollOffset;
 
     IntPoint m_cachedOverlayScrollbarOffset;
 
     // For areas with overflow, we have a pair of scrollbars.
     RefPtrWillBePersistent<Scrollbar> m_hBar;
     RefPtrWillBePersistent<Scrollbar> m_vBar;
 
     // Renderers to hold our custom scroll corner.
     RawPtrWillBePersistent<RenderScrollbarPart> m_scrollCorner;
 
     // Renderers to hold our custom resizer.
     RawPtrWillBePersistent<RenderScrollbarPart> m_resizer;
 };
 
 } // namespace blink
 
 #endif // RenderLayerScrollableArea_h