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

Source/core/rendering/RenderLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 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 852 matching lines @@
         horizontalScrollbar->setFrameRect(hBarRect);
     }
 
     const IntRect& scrollCorner = scrollCornerRect();
     if (m_scrollCorner)
         m_scrollCorner->setFrameRect(scrollCorner);
 
     if (m_resizer)
         m_resizer->setFrameRect(resizerCornerRect(borderBox, ResizerForPointer));
 
+    // FIXME, this should eventually be removed, once we are certain that composited
+    // controls get correctly positioned on a compositor update. For now, conservatively
+    // leaving this unchanged.
     if (m_box->compositedLayerMapping())
         m_box->compositedLayerMapping()->positionOverflowControlsLayers(offsetFromRoot);
 }
 
 void RenderLayerScrollableArea::updateScrollCornerStyle()
 {
     RenderObject* actualRenderer = rendererForScrollbar(m_box);
     RefPtr<RenderStyle> corner = m_box->hasOverflowClip() ? actualRenderer->getUncachedPseudoStyle(PseudoStyleRequest(SCROLLBAR_CORNER), actualRenderer->style()) : PassRefPtr<RenderStyle>(0);
     if (corner) {
         if (!m_scrollCorner) {
@@ skipping 386 matching lines @@
         baseHeight = baseHeight / zoomFactor;
         element->setInlineStyleProperty(CSSPropertyHeight, roundToInt(baseHeight + difference.height()), CSSPrimitiveValue::CSS_PX);
     }
 
     document.updateLayout();
 
     // FIXME (Radar 4118564): We should also autoscroll the window as necessary to keep the point under the cursor in view.
 }
 
 } // Namespace WebCore