Replace the current contiguity check for composited scrolling

Source/core/rendering/RenderLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 825 matching lines @@
 
     bool isDirtyStackingContainer() const { return m_zOrderListsDirty && isStackingContainer(); }
 
     void setAncestorChainHasSelfPaintingLayerDescendant();
     void dirtyAncestorChainHasSelfPaintingLayerDescendantStatus();
 
     void setAncestorChainHasOutOfFlowPositionedDescendant();
     void dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
 
     bool acceleratedCompositingForOverflowScrollEnabled() const;
-    void updateDescendantsAreContiguousInStackingOrder();
-    void updateDescendantsAreContiguousInStackingOrderRecursive(const HashMap<const RenderLayer*, int>&, int& minIndex, int& maxIndex, int& count, bool firstIteration);
+    void updateCanBeStackingContainer();
     void collectBeforePromotionZOrderList(RenderLayer* ancestorStackingContext, OwnPtr<Vector<RenderLayer*> >& posZOrderListBeforePromote, OwnPtr<Vector<RenderLayer*> >& negZOrderListBeforePromote);
     void collectAfterPromotionZOrderList(RenderLayer* ancestorStackingContext, OwnPtr<Vector<RenderLayer*> >& posZOrderListAfterPromote, OwnPtr<Vector<RenderLayer*> >& negZOrderListAfterPromote);
 
     void computeRepaintRects(const RenderLayerModelObject* repaintContainer, const RenderGeometryMap* = 0);
     void computeRepaintRectsIncludingDescendants();
     void clearRepaintRects();
 
     void clipToRect(RenderLayer* rootLayer, GraphicsContext*, const LayoutRect& paintDirtyRect, const ClipRect&,
                     BorderRadiusClippingRule = IncludeSelfForBorderRadius);
     void restoreClip(GraphicsContext*, const LayoutRect& paintDirtyRect, const ClipRect&);
@@ skipping 284 matching lines @@
     // may fall outside of our clip preventing things like opting into
     // composited scrolling (which causes clipping of all descendants).
     bool m_hasUnclippedDescendant : 1;
 
     ForceNeedsCompositedScrollingMode m_forceNeedsCompositedScrolling;
     bool m_needsCompositedScrolling : 1;
 
     // If this is true, then no non-descendant appears between any of our
     // descendants in stacking order. This is one of the requirements of being
     // able to safely become a stacking context.
-    bool m_descendantsAreContiguousInStackingOrder : 1;
-    bool m_descendantsAreContiguousInStackingOrderDirty : 1;
+    bool m_canBePromotedToStackingContainer : 1;
+    bool m_canBePromotedToStackingContainerDirty : 1;
 
     const bool m_isRootLayer : 1;
 
     bool m_usedTransparency : 1; // Tracks whether we need to close a transparent layer, i.e., whether
                                  // we ended up painting this layer or any descendants (and therefore need to
                                  // blend).
     bool m_paintingInsideReflection : 1;  // A state bit tracking if we are painting inside a replica.
     bool m_inOverflowRelayout : 1;
     unsigned m_repaintStatus : 2; // RepaintStatus
 
@@ skipping 161 matching lines @@
 
 } // namespace WebCore
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLayerTree(const WebCore::RenderLayer*);
 void showLayerTree(const WebCore::RenderObject*);
 #endif
 
 #endif // RenderLayer_h