Make composited scrolling codepaths co-operate.

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 828 matching lines @@
 
     void setAncestorChainHasSelfPaintingLayerDescendant();
     void dirtyAncestorChainHasSelfPaintingLayerDescendantStatus();
 
     void setAncestorChainHasOutOfFlowPositionedDescendant();
     void dirtyAncestorChainHasOutOfFlowPositionedDescendantStatus();
 
     bool acceleratedCompositingForOverflowScrollEnabled() const;
     // FIXME: This is a temporary flag and should be removed once accelerated
     // overflow scroll is ready (crbug.com/254111).
+    bool useCompositorDrivenAcceleratedScrolling() const;
     bool compositorDrivenAcceleratedScrollingEnabled() const;
     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 dirtyNormalFlowListCanBePromotedToStackingContainer();
     void dirtySiblingStackingContextCanBePromotedToStackingContainer();
 
     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&);
 
     bool shouldRepaintAfterLayout() const;
 
     void updateSelfPaintingLayer();
     void updateIsNormalFlowOnly();
     void updateVisibilityAfterStyleChange(const RenderStyle* oldStyle);
     void updateStackingContextsAfterStyleChange(const RenderStyle* oldStyle);
 
     void updateScrollbarsAfterStyleChange(const RenderStyle* oldStyle);
     void updateScrollbarsAfterLayout();
 
     void updateOutOfFlowPositioned(const RenderStyle* oldStyle);
 
     void setNeedsCompositedScrolling(bool);
+    void setNeedsToBeStackingContainer(bool needsToBeStackingContainer)
+    {
+        m_needsToBeStackingContainer = needsToBeStackingContainer;
+    }
     void didUpdateNeedsCompositedScrolling();
 
     // Returns true if the position changed.
     bool updateLayerPosition();
 
     void updateLayerPositions(RenderGeometryMap* = 0, UpdateLayerPositionsFlags = defaultFlags);
 
     enum UpdateLayerPositionsAfterScrollFlag {
         NoFlag = 0,
         IsOverflowScroll = 1 << 0,
@@ skipping 255 matching lines @@
 
     // This is true if we have an out-of-flow positioned descendant whose
     // containing block is our ancestor. If this is the case, the descendant
     // may fall outside of our clip preventing things like opting into
     // composited scrolling (which causes clipping of all descendants).
     unsigned m_hasUnclippedDescendant : 1;
 
     unsigned m_isUnclippedDescendant : 1;
 
     unsigned m_needsCompositedScrolling : 1;
+    unsigned m_needsToBeStackingContainer : 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.
     unsigned m_canBePromotedToStackingContainer : 1;
     unsigned m_canBePromotedToStackingContainerDirty : 1;
 
     const unsigned m_isRootLayer : 1;
 
     unsigned m_usedTransparency : 1; // Tracks whether we need to close a transparent layer, i.e., whether
@@ skipping 163 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