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

Source/core/style/ComputedStyleConstants.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 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 67 matching lines @@
 enum EBorderStyle { BNONE, BHIDDEN, INSET, GROOVE, OUTSET, RIDGE, DOTTED, DASHED, SOLID, DOUBLE };
 
 enum EBorderPrecedence { BOFF, BTABLE, BCOLGROUP, BCOL, BROWGROUP, BROW, BCELL };
 
 enum OutlineIsAuto { AUTO_OFF = 0, AUTO_ON };
 
 enum EPosition {
     StaticPosition = 0,
     RelativePosition = 1,
     AbsolutePosition = 2,
+    StickyPosition = 3,
     // This value is required to pack our bits efficiently in LayoutObject.
-    // FIXME: Is this still true now that we've remove position: sticky.
     FixedPosition = 6
 };
 
 enum EFloat {
     NoFloat, LeftFloat, RightFloat
 };
 
 enum EMarginCollapse { MCOLLAPSE, MSEPARATE, MDISCARD };
 
 // Box decoration attributes. Not inherited.
@@ skipping 438 matching lines @@
 
 enum ScrollSnapType {
     ScrollSnapTypeNone,
     ScrollSnapTypeMandatory,
     ScrollSnapTypeProximity
 };
 
 } // namespace blink
 
 #endif // ComputedStyleConstants_h