Fix overconstrained behaviour for position: sticky.

Source/core/page/scrolling/ScrollingConstraints.h

 /*
  * Copyright (C) 2012 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 41 matching lines @@
         , m_anchorEdges(other.m_anchorEdges)
     { }
 
     virtual ~ViewportConstraints() { }
 
     virtual ConstraintType constraintType() const = 0;
 
     AnchorEdges anchorEdges() const { return m_anchorEdges; }
     bool hasAnchorEdge(AnchorEdgeFlags flag) const { return m_anchorEdges & flag; }
     void addAnchorEdge(AnchorEdgeFlags edgeFlag) { m_anchorEdges |= edgeFlag; }
+    void removeAnchorEdge(AnchorEdgeFlags edgeFlag) { m_anchorEdges &= ~edgeFlag; }
     void setAnchorEdges(AnchorEdges edges) { m_anchorEdges = edges; }
 
     FloatSize alignmentOffset() const { return m_alignmentOffset; }
     void setAlignmentOffset(const FloatSize& offset) { m_alignmentOffset = offset; }
 
 protected:
     ViewportConstraints()
         : m_anchorEdges(0)
     { }
 
@@ skipping 108 matching lines @@
     float m_bottomOffset;
     FloatRect m_absoluteContainingBlockRect;
     FloatRect m_absoluteStickyBoxRect;
     FloatSize m_stickyOffsetAtLastLayout;
     FloatPoint m_layerPositionAtLastLayout;
 };
 
 } // namespace WebCore
 
 #endif // ScrollingConstraints_h