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

Source/core/style/ComputedStyleConstants.h

Index: Source/core/style/ComputedStyleConstants.h
diff --git a/Source/core/style/ComputedStyleConstants.h b/Source/core/style/ComputedStyleConstants.h
index edadd4146063d0e2f1952bd6f133178b6d0b0963..189aecbfcbe4cc5c1bd966cb331082d6b2e6cf9a 100644
--- a/Source/core/style/ComputedStyleConstants.h
+++ b/Source/core/style/ComputedStyleConstants.h
@@ -85,8 +85,8 @@ 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
 };