8329878: Reduce public interface of CardTableBarrierSet

Reviewed-by: tschatzl

Author: albertnetymk

src/hotspot/share/gc/shared/cardTableBarrierSet.hpp

@@ -45,10 +45,8 @@ class CardTableBarrierSet: public ModRefBarrierSet {
   // Some classes get to look at some private stuff.
   friend class VMStructs;
 
-public:
-
-  typedef CardTable::CardValue CardValue;
 protected:
+  typedef CardTable::CardValue CardValue;
   // Used in support of ReduceInitialCardMarks; only consulted if COMPILER2
   // or INCLUDE_JVMCI is being used
   bool       _defer_initial_card_mark;