c++ / mojo changes for 'external window mode'

services/ui/manifest.json

 {
   "name": "ui",
   "display_name": "UI Service",
   "interface_provider_specs": {
     "service_manager:connector": {
       "provides": {
         // A collection of interfaces needed by a generic client of mus.
         // Additional interfaces may be requested a-la-carte.
         // NOTE: when adding a new interface chances are you will want to add it
         // to window_manager as well.
@@ skipping 43 matching lines @@
           "ui::mojom::DisplayCompositor",
           "ui::mojom::DisplayManager",
           "ui::mojom::Gpu",
           "ui::mojom::IMEServer",
           "ui::mojom::InputDeviceServer",
           "ui::mojom::UserActivityMonitor",
           "ui::mojom::WindowManagerWindowTreeFactory"
         ],
         "window_tree_host_factory": [
           "ui::mojom::WindowTreeHostFactory"
+        ],
+        "window_tree_host_factory_registrar": [
+          "ui::mojom::WindowTreeHostFactoryRegistrar"
         ]
       },
       "requires": {
         "*": [ "app" ],
         "catalog": [ "app" ],
         "service_manager": [ "service_manager:all_users" ],
         "ui": [ "ozone" ]
       }
     }
   }
 }