SkMojo: test linking Skia against the Mojo SDK

TODO: build on systems other than Linux.

Add mojo_skd to the DEPS.

Add a DM::Via called `mojo-`.

everything is hidden behind the gyp variable `skia_mojo`.
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1644043003

Review URL: https://codereview.chromium.org/1644043003
diff --git a/dm/DMSrcSink.h b/dm/DMSrcSink.h
index a719bd1..3a7d4ee 100644
--- a/dm/DMSrcSink.h
+++ b/dm/DMSrcSink.h
@@ -357,6 +357,12 @@
     Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
 };
 
+class ViaMojo : public Via {
+public:
+    explicit ViaMojo(Sink* sink) : Via(sink) {}
+    Error draw(const Src&, SkBitmap*, SkWStream*, SkString*) const override;
+};
+
 }  // namespace DM
 
 #endif//DMSrcSink_DEFINED