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

Source/core/layout/LayoutObject.h

Index: Source/core/layout/LayoutObject.h
diff --git a/Source/core/layout/LayoutObject.h b/Source/core/layout/LayoutObject.h
index 462de417a2338903ebcd6edfa1c5ae86c83ecd41..7a4976477e845edb15cf6475212994acf71f2dca 100644
--- a/Source/core/layout/LayoutObject.h
+++ b/Source/core/layout/LayoutObject.h
@@ -506,7 +506,9 @@ public:
     bool isFloating() const { return m_bitfields.floating(); }
 
     bool isOutOfFlowPositioned() const { return m_bitfields.isOutOfFlowPositioned(); } // absolute or fixed positioning
+    bool isInFlowPositioned() const { return m_bitfields.isInFlowPositioned(); } // relative or sticky positioning
     bool isRelPositioned() const { return m_bitfields.isRelPositioned(); } // relative positioning
+    bool isStickyPositioned() const { return m_bitfields.isStickyPositioned(); } // sticky positioning
     bool isPositioned() const { return m_bitfields.isPositioned(); }
 
     bool isText() const  { return m_bitfields.isText(); }
@@ -1279,6 +1281,7 @@ private:
             IsStaticallyPositioned = 0,
             IsRelativelyPositioned = 1,
             IsOutOfFlowPositioned = 2,
+            IsStickyPositioned = 3,
         };
 
     public:
@@ -1388,6 +1391,8 @@ private:
     public:
         bool isOutOfFlowPositioned() const { return m_positionedState == IsOutOfFlowPositioned; }
         bool isRelPositioned() const { return m_positionedState == IsRelativelyPositioned; }
+        bool isStickyPositioned() const { return m_positionedState == IsStickyPositioned; }
+        bool isInFlowPositioned() const { return m_positionedState == IsRelativelyPositioned || m_positionedState == IsStickyPositioned; }
         bool isPositioned() const { return m_positionedState != IsStaticallyPositioned; }
 
         void setPositionedState(int positionState)