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

Source/core/css/CSSPrimitiveValueMappings.h

 /*
  * Copyright (C) 2007 Alexey Proskuryakov <ap@nypop.com>.
  * Copyright (C) 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2009 Jeff Schiller <codedread@gmail.com>
  * Copyright (C) Research In Motion Limited 2010. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
@@ skipping 1821 matching lines @@
         break;
     case RelativePosition:
         m_value.valueID = CSSValueRelative;
         break;
     case AbsolutePosition:
         m_value.valueID = CSSValueAbsolute;
         break;
     case FixedPosition:
         m_value.valueID = CSSValueFixed;
         break;
+    case StickyPosition:
+        m_value.valueID = CSSValueSticky;
+        break;
     }
 }
 
 template<> inline CSSPrimitiveValue::operator EPosition() const
 {
     ASSERT(isValueID());
     switch (m_value.valueID) {
     case CSSValueStatic:
         return StaticPosition;
     case CSSValueRelative:
         return RelativePosition;
     case CSSValueAbsolute:
         return AbsolutePosition;
     case CSSValueFixed:
         return FixedPosition;
+    case CSSValueSticky:
+        return StickyPosition;
     default:
         break;
     }
 
     ASSERT_NOT_REACHED();
     return StaticPosition;
 }
 
 template<> inline CSSPrimitiveValue::CSSPrimitiveValue(EResize e)
     : CSSValue(PrimitiveClass)
@@ skipping 2806 matching lines @@
     default:
         break;
     }
     ASSERT_NOT_REACHED();
     return ScrollSnapTypeNone;
 }
 
 } // namespace blink
 
 #endif