Clear the groupedMapping parameter in layers when removed from the group

Source/core/layout/compositing/CompositingLayerAssigner.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google 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
@@ skipping 192 matching lines @@
 
         // Issue a paint invalidation, since |layer| may have been added to an already-existing squashing layer.
         TRACE_LAYER_INVALIDATION(layer, InspectorLayerInvalidationTrackingEvent::AddedToSquashingLayer);
         layersNeedingPaintInvalidation.append(layer);
         m_layersChanged = true;
     } else if (compositedLayerUpdate == RemoveFromSquashingLayer) {
         if (layer->groupedMapping()) {
             // Before removing |layer| from an already-existing squashing layer that may have other content, issue a paint invalidation.
             m_compositor->paintInvalidationOnCompositingChange(layer);
             layer->groupedMapping()->setNeedsGraphicsLayerUpdate(GraphicsLayerUpdateSubtree);
-            layer->setGroupedMapping(nullptr);
+            layer->setGroupedMapping(nullptr, true);
         }
 
         // If we need to issue paint invalidations, do so now that we've removed it from a squashed layer.
         TRACE_LAYER_INVALIDATION(layer, InspectorLayerInvalidationTrackingEvent::RemovedFromSquashingLayer);
         layersNeedingPaintInvalidation.append(layer);
         m_layersChanged = true;
 
         layer->setLostGroupedMapping(false);
     }
 }
@@ skipping 75 matching lines @@
 
     DeprecatedPaintLayerStackingNodeIterator iterator(*layer->stackingNode(), NormalFlowChildren | PositiveZOrderChildren);
     while (DeprecatedPaintLayerStackingNode* curNode = iterator.next())
         assignLayersToBackingsInternal(curNode->layer(), squashingState, layersNeedingPaintInvalidation);
 
     if (squashingState.hasMostRecentMapping && &squashingState.mostRecentMapping->owningLayer() == layer)
         squashingState.haveAssignedBackingsToEntireSquashingLayerSubtree = true;
 }
 
 }