mus: Make --use-mus-in-renderer work again.

content/public/app/mojo/content_browser_manifest.json

 {
   "manifest_version": 1,
   "name": "exe:content_browser",
   "process-group": "browser",
   "display_name": "Content Browser",
   "capabilities": {
     "required": {
       "*": { "classes": [ "app" ] },
+      "exe:content_renderer": {
+        "interfaces": [ "content::mojom::RenderWidgetWindowTreeClientFactory" ]
+      },
       "mojo:shell": {
         "classes": [
           "shell:client_process",
           "shell:instance_name",
           "shell:user_id"
         ]
       },
       "mojo:user": {
         "interfaces": [
           "leveldb::mojom::LevelDBService",
           "user_service::UserService"
          ]
       }
     }
   }
 }