Hook up Chrome's wallpaper picker for mash.

ash/mus/manifest.json

 {
   "manifest_version":  1,
   "name": "mojo:ash_sysui",
   "display_name": "System UI",
   "capabilities": {
     "required": {
       "*": { "classes": [ "app" ] },
       "mojo:desktop_wm": {
         "interfaces": [
           "mash::wm::mojom::ShelfLayout",
           "mash::wm::mojom::UserWindowController"
         ]
       },
-      "exe:chrome" : { "classes": [ "app_list:presenter" ] }
+      "exe:chrome" : {
+        "interfaces": [ "mash::mojom::WallpaperHelper" ],
+        "classes": [ "app_list:presenter" ]
+      }
     }
   }
 }