Move Mutexy things to private.

There is no API change.

TBR=reed@google.com

BUG=skia:

Review URL: https://codereview.chromium.org/1368333004
diff --git a/bench/RefCntBench.cpp b/bench/RefCntBench.cpp
index 1168055..313e9a4 100644
--- a/bench/RefCntBench.cpp
+++ b/bench/RefCntBench.cpp
@@ -6,7 +6,7 @@
  */
 #include <memory>
 #include "Benchmark.h"
-#include "../private/SkAtomics.h"
+#include "SkAtomics.h"
 #include "SkRefCnt.h"
 #include "SkWeakRefCnt.h"