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

Source/core/css/CSSPrimitiveValueMappings.h

Index: Source/core/css/CSSPrimitiveValueMappings.h
diff --git a/Source/core/css/CSSPrimitiveValueMappings.h b/Source/core/css/CSSPrimitiveValueMappings.h
index c87dae894140b94a4f71767e741826f3635bdacc..ca2c5574d01e32d92712938b5ce14a535201951f 100644
--- a/Source/core/css/CSSPrimitiveValueMappings.h
+++ b/Source/core/css/CSSPrimitiveValueMappings.h
@@ -1839,6 +1839,9 @@ template<> inline CSSPrimitiveValue::CSSPrimitiveValue(EPosition e)
     case FixedPosition:
         m_value.valueID = CSSValueFixed;
         break;
+    case StickyPosition:
+        m_value.valueID = CSSValueSticky;
+        break;
     }
 }
 
@@ -1854,6 +1857,8 @@ template<> inline CSSPrimitiveValue::operator EPosition() const
         return AbsolutePosition;
     case CSSValueFixed:
         return FixedPosition;
+    case CSSValueSticky:
+        return StickyPosition;
     default:
         break;
     }