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

Source/core/rendering/RenderLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. 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>
@@ skipping 181 matching lines @@
         UpdatePagination = 1 << 4
     };
     typedef unsigned UpdateLayerPositionsFlags;
     static const UpdateLayerPositionsFlags defaultFlags = CheckForRepaint | IsCompositingUpdateRoot | UpdateCompositingLayers;
 
     void updateLayerPositionsAfterLayout(const RenderLayer* rootLayer, UpdateLayerPositionsFlags);
 
     void updateLayerPositionsAfterOverflowScroll();
     void updateLayerPositionsAfterDocumentScroll();
 
-    void positionNewlyCreatedOverflowControls();
 
     bool isPaginated() const { return m_isPaginated; }
     RenderLayer* enclosingPaginationLayer() const { return m_enclosingPaginationLayer; }
 
     void updateTransform();
 
     void updateBlendMode();
 
     const LayoutSize& offsetForInFlowPosition() const { return m_offsetForInFlowPosition; }
 
@@ skipping 561 matching lines @@
     void parentClipRects(const ClipRectsContext&, ClipRects&) const;
     ClipRect backgroundClipRect(const ClipRectsContext&) const;
 
     LayoutRect paintingExtent(const RenderLayer* rootLayer, const LayoutRect& paintDirtyRect, PaintBehavior);
 
     RenderLayer* enclosingTransformedAncestor() const;
 
     // 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();
 
     // FIXME: Temporary. Remove when new columns come online.
     bool useRegionBasedColumns() const;
 
     bool hasCompositingDescendant() const { return m_compositingProperties.hasCompositingDescendant; }
     void setHasCompositingDescendant(bool b)  { m_compositingProperties.hasCompositingDescendant = b; }
 
@@ skipping 195 matching lines @@
 
 } // namespace WebCore
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLayerTree(const WebCore::RenderLayer*);
 void showLayerTree(const WebCore::RenderObject*);
 #endif
 
 #endif // RenderLayer_h