When running DM, write test failures to json.

dm/DMJsonWriter.h

Index: dm/DMJsonWriter.h
diff --git a/dm/DMJsonWriter.h b/dm/DMJsonWriter.h
index c358fdd9237285779294489964ade172c11305d1..66ce530624b36841b833fa9d999aea8768715922 100644
--- a/dm/DMJsonWriter.h
+++ b/dm/DMJsonWriter.h
@@ -9,6 +9,7 @@
 #define DMJsonWriter_DEFINED
 
 #include "SkString.h"
+#include "Test.h"
 
 namespace DM {
 
@@ -35,6 +36,11 @@ public:
     static void AddBitmapResult(const BitmapResult&);
 
     /**
+     *  Add a Failure from a Test.
+     */
+    static void AddTestFailure(const skiatest::Failure&);
+
+    /**
      *  Write all collected results to the file FLAGS_writePath[0]/dm.json.
      */
     static void DumpJson();