RLSA needsLayout on customscrollbar thickness change.

Source/core/rendering/RenderLayerScrollableArea.h

Index: Source/core/rendering/RenderLayerScrollableArea.h
diff --git a/Source/core/rendering/RenderLayerScrollableArea.h b/Source/core/rendering/RenderLayerScrollableArea.h
index 2d983c06fbbe36750a1448c6060b96b0e5354da8..06477c4e3b6dad035cb263e12f203d25ef8ae9ee 100644
--- a/Source/core/rendering/RenderLayerScrollableArea.h
+++ b/Source/core/rendering/RenderLayerScrollableArea.h
@@ -129,6 +129,8 @@ public:
 
     bool hasScrollbar() const { return m_hBar || m_vBar; }
 
+    bool didCustomScrollbarRectChanged();
+
     // FIXME: This should be removed.
     bool hasScrollCorner() const { return m_scrollCorner; }
 
@@ -238,6 +240,9 @@ private:
     // no longer need this bit.
     unsigned m_needsCompositedScrolling : 1;
 
+    unsigned m_horizontalScrollbarChanged : 1;
+    unsigned m_verticalScrollbarChanged : 1;
+
     // The width/height of our scrolled area.
     LayoutRect m_overflowRect;