Squashed fixed position layers shouldn't flicker while scrolling

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index 6f43339a2d51e1a8375cdc72e242075f8ac32c49..c01050fe422f690e5aa17cc83b14fcad14e974ef 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -212,9 +212,10 @@ void ScrollingCoordinator::updateLayerPositionConstraint(RenderLayer* layer)
 {
     ASSERT(layer->hasCompositedLayerMapping());
     CompositedLayerMapping* compositedLayerMapping = layer->compositedLayerMapping();
-    GraphicsLayer* mainLayer = compositedLayerMapping->localRootForOwningLayer();
+    GraphicsLayer* mainLayer = compositedLayerMapping->childForSuperlayers();
 
     // Avoid unnecessary commits
+    clearPositionConstraintExceptForLayer(compositedLayerMapping->squashingContainmentLayer(), mainLayer);
     clearPositionConstraintExceptForLayer(compositedLayerMapping->ancestorClippingLayer(), mainLayer);
     clearPositionConstraintExceptForLayer(compositedLayerMapping->mainGraphicsLayer(), mainLayer);