Defer the real work in updateCompositingLayers until it's really needed.

Source/core/rendering/RenderLayer.h

Index: Source/core/rendering/RenderLayer.h
diff --git a/Source/core/rendering/RenderLayer.h b/Source/core/rendering/RenderLayer.h
index a49c2f86ea2a8532437955d427d68764c2aaeea9..42c55bab2e4e7965cad49bb7349e50ca76f9bda2 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -199,7 +199,6 @@ public:
     void updateLayerPositionsAfterOverflowScroll();
     void updateLayerPositionsAfterDocumentScroll();
 
-    void positionNewlyCreatedOverflowControls();
 
     bool isPaginated() const { return m_isPaginated; }
     RenderLayer* enclosingPaginationLayer() const { return m_enclosingPaginationLayer; }
@@ -781,6 +780,9 @@ private:
     // Convert a point in absolute coords into layer coords, taking transforms into account
     LayoutPoint absoluteToContents(const LayoutPoint&) const;
 
+    // If IntSize is not given, then we must incur additional overhead to instantiate a RenderGeometryMap
+    // and compute the correct offset ourselves.
+    void positionOverflowControls();
     void positionOverflowControls(const IntSize&);
 
     void updatePagination();