ash/mash: Add a mus-client that sets up ash to provide the system ui for mash.

mash/shell/BUILD.gn

Index: mash/shell/BUILD.gn
diff --git a/mash/shell/BUILD.gn b/mash/shell/BUILD.gn
index 0c2c43a5545b89d3b9047ce3609ff0f60382f468..16adaeaefbf27ea9e4d65f857778424fd6e69c8a 100644
--- a/mash/shell/BUILD.gn
+++ b/mash/shell/BUILD.gn
@@ -34,6 +34,10 @@ mojo_native_application("shell") {
     "//mash/wallpaper",
     "//mash/wm",
   ]
+
+  if (use_ash) {
+    data_deps += [ "//ash/mus" ]
+  }
 }
 
 mojo_application_manifest("manifest") {