Port SkRefCnt.h to new SkAtomics.h

include/core/SkAtomics.h

Index: include/core/SkAtomics.h
diff --git a/include/core/SkAtomics.h b/include/core/SkAtomics.h
index 1a84049970ddbe97ce8fdca5c890080ee27d64c9..e62d6e093155204dbbae14c3ccb9b7fe2d24eddf 100644
--- a/include/core/SkAtomics.h
+++ b/include/core/SkAtomics.h
@@ -34,6 +34,11 @@ bool sk_atomic_compare_exchange(T*, T* expected, T desired,
     #include "../ports/SkAtomics_sync.h"
 #endif
 
+inline void sk_memory_barrier(sk_memory_order mo) {
+    int junk;
+    (void)sk_atomic_fetch_add(&junk, 42, mo);
+}
+
 // From here down we have shims for our old atomics API, to be weaned off of.
 // We use the default sequentially-consistent memory order to make things simple
 // and to match the practical reality of our old _sync and _win implementations.