Introduce Display Compositor mojo interface. Use InProcessContextProvider.

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.
         "app": [
           "ui::mojom::Clipboard",
+          "ui::mojom::DisplayCompositor",
           "ui::mojom::DisplayManager",
           "ui::mojom::Gpu",
           "ui::mojom::GpuService",
           "ui::mojom::IMEServer",
           "ui::mojom::InputDeviceServer",
           "ui::mojom::WindowTreeFactory"
         ],
         // Interfaces provided by mus-gpu for mus-ws.
         "ozone": [
           "ui::ozone::mojom::DeviceCursor"
@@ skipping 23 matching lines @@
       },
       "requires": {
         "*": [ "app" ],
         "catalog": [ "catalog:catalog" ],
         "service_manager": [ "service_manager:all_users" ],
         "ui": [ "ozone" ]
       }
     }
   }
 }