Avoid tree walks when computing RenderLayer::scrollParent

Source/core/rendering/compositing/GraphicsLayerUpdater.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 129 matching lines @@
     if (!depth) {
         int percentageIncreaseInPixels = static_cast<int>(m_pixelsAddedByPromotingAllTransitions / m_pixelsWithoutPromotingAllTransitions * 100);
         blink::Platform::current()->histogramCustomCounts("Renderer.PixelIncreaseFromTransitions", percentageIncreaseInPixels, 0, 1000, 50);
     }
 }
 
 // This just updates layer geometry without changing the hierarchy.
 void GraphicsLayerUpdater::updateRecursive(RenderLayer& layer, UpdateType updateType)
 {
     updateType = update(layer, updateType);
+    layer.clearAncestorDependentPropertyCache();
 
 #if !ASSERT_DISABLED
     LayerListMutationDetector mutationChecker(layer.stackingNode());
 #endif
 
     RenderLayerStackingNodeIterator iterator(*layer.stackingNode(), AllChildren);
     while (RenderLayerStackingNode* curNode = iterator.next())
         updateRecursive(*curNode->layer(), updateType);
 }
 
@@ skipping 20 matching lines @@
     if (!layer.parent())
         layer.compositor()->updateRootLayerPosition();
 
     if (mapping->hasUnpositionedOverflowControlsLayers())
         layer.scrollableArea()->positionOverflowControls();
 
     return updateType;
 }
 
 } // namespace WebCore