Reify view ownership as a message pipe.

mojo/services/ui/views/interfaces/view_provider.mojom

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [DartPackage="mojo_services"]
 module mojo.ui;
 
 import "mojo/public/interfaces/application/service_provider.mojom";
 import "mojo/services/ui/views/interfaces/views.mojom";
 
 // Provides a View upon request.
 //
 // Applications should implement and expose this service so that they can
 // expose views to be embedded into other applications.
 [ServiceName="mojo::ui::ViewProvider"]
 interface ViewProvider {
   // Creates and registers a view with the view manager and returns its
-  // view token (as provided by |ViewManager.RegisterView()|).
+  // view owner which may subsequently be passed to |ViewHost.AddChild()|
+  // to attach the view to a view hierarchy.
   //
-  // Having received the view token, the caller should attach the view to
-  // a view tree and lay it out.
+  // Implementors of this interface are responsible for creating the view
+  // and forwarding the |view_owner| interface request to
+  // |ViewManager.RegisterView()|.
   //
   // The caller may provide services to the view via the |services|
   // service provider.
   //
   // The caller may receive services from the view via the |exposed_services|
   // service provider.
-  CreateView(mojo.ServiceProvider&? services,
-             mojo.ServiceProvider? exposed_services) =>
-                 (ViewToken view_token);
+  CreateView(ViewOwner& view_owner,
+             mojo.ServiceProvider&? services,
+             mojo.ServiceProvider? exposed_services);
 };