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/DMSrcSink.h b/dm/DMSrcSink.h
index bf8e7d6..1fdc9ed 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -130,6 +130,16 @@
     const char* fileExtension() const SK_OVERRIDE { return "skp"; }
 };
 
+class SVGSink : public Sink {
+public:
+    SVGSink();
+
+    Error draw(const Src&, SkBitmap*, SkWStream*) const SK_OVERRIDE;
+    int enclave() const SK_OVERRIDE { return kAnyThread_Enclave; }
+    const char* fileExtension() const SK_OVERRIDE { return "svg"; }
+};
+
+
 /*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
 
 class ViaMatrix : public Sink {