Update scroll parent correctly for squashing layers.

Source/core/rendering/compositing/CompositedLayerMapping.cpp

Index: Source/core/rendering/compositing/CompositedLayerMapping.cpp
diff --git a/Source/core/rendering/compositing/CompositedLayerMapping.cpp b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
index fcdf3a996d4f36100e1656bad5a6fc1413d103a9..f87dec876fbb6569295360bae854c089e076cd4f 100644
--- a/Source/core/rendering/compositing/CompositedLayerMapping.cpp
+++ b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
@@ -463,6 +463,8 @@ bool CompositedLayerMapping::updateGraphicsLayerConfiguration()
     if (m_owningLayer.needsCompositedScrolling())
         needsDescendentsClippingLayer = false;
 
+    // FIXME: this should not be computed dynamically, but passed down during the compositing update.
+    // See http://crbug.com/351851
     RenderLayer* scrollParent = m_owningLayer.scrollParent();
     bool needsAncestorClip = compositor->clippedByAncestor(&m_owningLayer);
     if (scrollParent) {
@@ -870,6 +872,9 @@ GraphicsLayerUpdater::UpdateType CompositedLayerMapping::updateGraphicsLayerGeom
     updateRenderingContext();
     updateShouldFlattenTransform();
     updateChildrenTransform();
+    // FIXME: this should not be computed dynamically, but passed down during the compositing update.
+    // See http://crbug.com/351851
+    updateScrollParent(m_owningLayer.scrollParent());
     registerScrollingLayers();
 
     updateCompositingReasons();
@@ -1396,9 +1401,12 @@ static void updateScrollParentForGraphicsLayer(GraphicsLayer* layer, GraphicsLay
 
 void CompositedLayerMapping::updateScrollParent(RenderLayer* scrollParent)
 {
+    if (!scrollParent && m_squashedLayers.size())
+        scrollParent = m_squashedLayers[0].renderLayer->scrollParent();
 
     if (ScrollingCoordinator* scrollingCoordinator = scrollingCoordinatorFromLayer(m_owningLayer)) {
-        GraphicsLayer* topmostLayer = localRootForOwningLayer();
+        GraphicsLayer* topmostLayer = childForSuperlayers();
+        updateScrollParentForGraphicsLayer(m_squashingContainmentLayer.get(), topmostLayer, scrollParent, scrollingCoordinator);
         updateScrollParentForGraphicsLayer(m_ancestorClippingLayer.get(), topmostLayer, scrollParent, scrollingCoordinator);
         updateScrollParentForGraphicsLayer(m_graphicsLayer.get(), topmostLayer, scrollParent, scrollingCoordinator);
     }