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/tests/skia_test.cpp b/tests/skia_test.cpp
index 4230935..dc5ae97 100644
--- a/tests/skia_test.cpp
+++ b/tests/skia_test.cpp
@@ -8,7 +8,7 @@
 #include "CrashHandler.h"
 #include "OverwriteLine.h"
 #include "Resources.h"
-#include "../private/SkAtomics.h"
+#include "SkAtomics.h"
 #include "SkCommonFlags.h"
 #include "SkGraphics.h"
 #include "SkOSFile.h"