Use mojo Shelf interfaces for both mash and classic ash.

ash/mus/manifest.json

 {
   "manifest_version":  1,
   "name": "service:ash",
   "display_name": "Ash Window Manager and Shell",
   "capabilities": {
     "provided": {
+      "ash": [
+        "ash::mojom::ShelfController",
+        "ash::mojom::SystemTray",
+        "ash::mojom::WallpaperController"
+      ],
       "mus:window_manager" : [ "ui::mojom::AcceleratorRegistrar" ]
     },
     "required": {
       "*": { "classes": [ "accessibility", "app" ] },
       "service:content_browser" : {
         "interfaces": [
           "ash::mojom::SystemTrayClient",
           "ash::mojom::WallpaperManager",
           "app_list::mojom::AppListPresenter",
           "keyboard::mojom::Keyboard"
@@ skipping 10 matching lines @@
         ]
       },
       "service:touch_hud": {
         "interfaces": [
           "mash::mojom::Launchable"
         ]
       }
     }
   }
 }