Update DM to allow Src's to have optional options.

Review URL: https://codereview.chromium.org/1059513002
diff --git a/dm/DMJsonWriter.h b/dm/DMJsonWriter.h
index 5934846..67c9cf6 100644
--- a/dm/DMJsonWriter.h
+++ b/dm/DMJsonWriter.h
@@ -26,6 +26,7 @@
         SkString name;            // E.g. "ninepatch-stretch", "desk_gws.skp"
         SkString config;          //      "gpu", "8888", "serialize", "pipe"
         SkString sourceType;      //      "gm", "skp", "image"
+        SkString sourceOptions;   //      "image", "codec", "subset", "scanline"
         SkString md5;             // In ASCII, so 32 bytes long.
         SkString ext;             // Extension of file we wrote: "png", "pdf", ...
     };