SVG backend in DM

Not enabled by default, but this should get you SKPs, GMs etc for free to play with.

$ out/Debug/dm -w svgs --src gm skp --config svg

BUG=skia:

Review URL: https://codereview.chromium.org/892693002
diff --git a/dm/DM.cpp b/dm/DM.cpp
index f087a32..c8d1ec7 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -221,7 +221,8 @@
         SINK("8888", RasterSink, kN32_SkColorType);
         SINK("pdf",  PDFSink);
         SINK("skp",  SKPSink);
-        SINK("null",  NullSink);
+        SINK("svg",  SVGSink);
+        SINK("null", NullSink);
     }
 #undef SINK
     return NULL;