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/DEPS b/DEPS
index 7145953..80c6b18 100644
--- a/DEPS
+++ b/DEPS
@@ -47,6 +47,9 @@
 
   # microhttpd for skiaserve
   "third_party/externals/microhttpd" : "https://android.googlesource.com/platform/external/libmicrohttpd@748945ec6f1c67b7efc934ab0808e1d32f2fb98d",
+
+  # Mojo SDK for Mojo RPC experiments.
+  "third_party/externals/mojo/public" : "https://chromium.googlesource.com/external/github.com/domokit/mojo_sdk@172be18ea745ab29aea132e6cb952e726dc32543",
 }
 
 recursedeps = [ "common" ]