WIP: simplified display management in ash

chrome/app/mash/chrome_mus_manifest.json

 {
   // NOTE: This is the manifest for service:chrome_mus, which is only used to
   // package services for the Mus environment when baked into the Chrome binary
   // (i.e. "chrome --mus")
   //
   // If you need to require or provide capabilities from Chrome itself within
   // the you should instead modify chrome_content_browser_manifest_overlay.json.
   "name": "chrome_mus",
   "display_name": "Chrome Mus Packaged Services",
   "interface_provider_specs": {
     "service_manager:connector": {
       "provides": {
         "service_manager:service_factory": [
           "service_manager::mojom::ServiceFactory"
         ]
       },
       "requires": {
         "*": [ "app" ],
         "catalog": [ "control" ],
         "content_browser": [ "app" ],
         "service_manager": [ "service_manager:instance_per_child" ],
-        "ui": [ "app" ]
+        "ui": [ "window_manager" ]
       }
     }
   }
 }