Squashed fixed position layers shouldn't flicker while scrolling

Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 194 matching lines @@
         // Composited layers that inherit a fixed position state will be positioned with respect to the nearest compositedLayerMapping's GraphicsLayer.
         // So, once we find a layer that has its own compositedLayerMapping, we can stop searching for a fixed position RenderObject.
     } while (layer && !layer->hasCompositedLayerMapping());
     return WebLayerPositionConstraint();
 }
 
 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);
 
     if (WebLayer* scrollableLayer = toWebLayer(mainLayer))
         scrollableLayer->setPositionConstraint(computePositionConstraint(layer));
 }
 
 void ScrollingCoordinator::willDestroyScrollableArea(ScrollableArea* scrollableArea)
 {
     removeWebScrollbarLayer(scrollableArea, HorizontalScrollbar);
@@ skipping 719 matching lines @@
     bool frameIsScrollable = frameView && frameView->isScrollable();
     if (frameIsScrollable != m_wasFrameScrollable)
         return true;
 
     if (WebLayer* scrollLayer = frameView ? toWebLayer(frameView->layerForScrolling()) : 0)
         return blink::WebSize(frameView->contentsSize()) != scrollLayer->bounds();
     return false;
 }
 
 } // namespace blink