Add methods to RenderLayer to get at the painting or scrolling graphics layers, if present.

Source/core/rendering/RenderLayer.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 3320 matching lines @@
         return true;
     return renderer()->document().lifecycle().state() >= DocumentLifecycle::InCompositingUpdate;
 }
 
 CompositedLayerMappingPtr RenderLayer::compositedLayerMapping() const
 {
     ASSERT(isAllowedToQueryCompositingState());
     return m_compositedLayerMapping.get();
 }
 
+GraphicsLayer* RenderLayer::graphicsLayerBacking() const
+{
+    switch (compositingState()) {
+    case NotComposited:
+        return 0;
+    case PaintsIntoGroupedBacking:
+        return groupedMapping()->squashingLayer();
+    default:
+        return compositedLayerMapping()->mainGraphicsLayer();
+    }
+}
+
+GraphicsLayer* RenderLayer::graphicsLayerBackingForScrolling() const
+{
+    switch (compositingState()) {
+    case NotComposited:
+        return 0;
+    case PaintsIntoGroupedBacking:
+        return groupedMapping()->squashingLayer();
+    default:
+        return compositedLayerMapping()->scrollingContentsLayer() ? compositedLayerMapping()->scrollingContentsLayer() : compositedLayerMapping()->mainGraphicsLayer();
+    }
+}
+
 CompositedLayerMappingPtr RenderLayer::ensureCompositedLayerMapping()
 {
     if (!m_compositedLayerMapping) {
         m_compositedLayerMapping = adoptPtr(new CompositedLayerMapping(*this));
         m_compositedLayerMapping->setNeedsGraphicsLayerUpdate(GraphicsLayerUpdateSubtree);
 
         updateOrRemoveFilterEffectRenderer();
 
         if (RuntimeEnabledFeatures::cssCompositingEnabled())
             compositedLayerMapping()->setBlendMode(m_blendInfo.blendMode());
@@ skipping 411 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif