Fold gmtoskp into DM, as --src gm --config skp.

BUG=skia:

Review URL: https://codereview.chromium.org/885733002
diff --git a/dm/DM.cpp b/dm/DM.cpp
index 5b99a4c..900eb58 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -208,6 +208,7 @@
         SINK("565",  RasterSink, kRGB_565_SkColorType);
         SINK("8888", RasterSink, kN32_SkColorType);
         SINK("pdf",  PDFSink);
+        SINK("skp",  SKPSink);
     }
 #undef SINK
     return NULL;