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..ac460b5825ffd54c8ee995681dc1ebb5b2e08b63 100644
--- a/Source/core/rendering/compositing/CompositedLayerMapping.cpp
+++ b/Source/core/rendering/compositing/CompositedLayerMapping.cpp
@@ -870,6 +870,7 @@ GraphicsLayerUpdater::UpdateType CompositedLayerMapping::updateGraphicsLayerGeom
     updateRenderingContext();
     updateShouldFlattenTransform();
     updateChildrenTransform();
+    updateScrollParent(m_owningLayer.scrollParent());

[abarth-chromium, 2014/03/12 18:45:40]

This function walks up the RenderLayer tree.  Instead, we should remember this
information as we walk down the RenderLayer tree.

[Ian Vollick, 2014/03/12 20:03:32]

Agreed, though it's a little tricky since a layer's scroll parent depends on its
containing block. We'll need to pass down a collection of scroll parents for the
variously positioned descendants (like we do for RenderGeometryMap). This is a
biggish refactor and I'm hoping I can address it in a separate patch. I've
created a bug (http://crbug.com/351851) and added FIXMES if you're ok with that.

     registerScrollingLayers();
 
     updateCompositingReasons();
@@ -1396,9 +1397,12 @@ 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);
     }