Add position: sticky as supported position value when CSSStickyPosition is enabled.

Source/core/style/ComputedStyle.h

 /*
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 448 matching lines @@
 
     // Whether or not a positioned element requires normal flow x/y to be computed
     // to determine its position.
     bool hasAutoLeftAndRight() const { return left().isAuto() && right().isAuto(); }
     bool hasAutoTopAndBottom() const { return top().isAuto() && bottom().isAuto(); }
     bool hasStaticInlinePosition(bool horizontal) const { return horizontal ? hasAutoLeftAndRight() : hasAutoTopAndBottom(); }
     bool hasStaticBlockPosition(bool horizontal) const { return horizontal ? hasAutoTopAndBottom() : hasAutoLeftAndRight(); }
 
     EPosition position() const { return static_cast<EPosition>(noninherited_flags.position); }
     bool hasOutOfFlowPosition() const { return position() == AbsolutePosition || position() == FixedPosition; }
-    bool hasInFlowPosition() const { return position() == RelativePosition; }
+    bool hasInFlowPosition() const { return position() == RelativePosition || position() == StickyPosition; }
     bool hasViewportConstrainedPosition() const { return position() == FixedPosition; }
     EFloat floating() const { return static_cast<EFloat>(noninherited_flags.floating); }
 
     const Length& width() const { return m_box->width(); }
     const Length& height() const { return m_box->height(); }
     const Length& minWidth() const { return m_box->minWidth(); }
     const Length& maxWidth() const { return m_box->maxWidth(); }
     const Length& minHeight() const { return m_box->minHeight(); }
     const Length& maxHeight() const { return m_box->maxHeight(); }
 
@@ skipping 1455 matching lines @@
 }
 
 inline bool ComputedStyle::hasPseudoElementStyle() const
 {
     return noninherited_flags.pseudoBits & PSEUDO_ELEMENT_MASK;
 }
 
 } // namespace blink
 
 #endif // ComputedStyle_h