DM::NullSink

Motivation:  The null sink can act as a control for experiments.

Review URL: https://codereview.chromium.org/873723007
diff --git a/dm/DMJsonWriter.cpp b/dm/DMJsonWriter.cpp
index 56c1301..8f1fa7e 100644
--- a/dm/DMJsonWriter.cpp
+++ b/dm/DMJsonWriter.cpp
@@ -75,6 +75,7 @@
     }
 
     SkString path = SkOSPath::Join(FLAGS_writePath[0], "dm.json");
+    sk_mkdir(FLAGS_writePath[0]);
     SkFILEWStream stream(path.c_str());
     stream.writeText(Json::StyledWriter().write(root).c_str());
     stream.flush();