Add a headless configuration to Mojo's native viewport service

mojo/services/public/interfaces/native_viewport/native_viewport.mojom

 // Copyright 2014 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.
 
 import "mojo/services/gles2/command_buffer.mojom"
 import "mojo/services/public/interfaces/geometry/geometry.mojom"
 import "mojo/services/public/interfaces/input_events/input_events.mojom"
 import "mojo/services/public/interfaces/surfaces/surface_id.mojom"
 
 module mojo {
 
 [Client=NativeViewportClient]
 interface NativeViewport {
   Create(Size size);
   Show();
   Hide();
   Close();
   SetBounds(Size size);
   SubmittedFrame(SurfaceId surface_id);
 };
 
 interface NativeViewportClient {
   OnCreated(uint64 native_viewport_id);
   OnBoundsChanged(Size size);
   OnDestroyed();
   OnEvent(Event event) => ();
 };
 
-// Connect to this interface before any other connections are made and call
-// UseTestConfig(), blocking on the reply. This will ensure that the correct
-// global initialization is done before subsequent connections.
+// Connect to this interface before any other connections are made to configure
+// the NativeViewport service.
 interface NativeViewportConfig {
+  // Call UseTestConfig() and block on the reply. This will ensure that the
+  // correct global initialization is done before subsequent connections.
   UseTestConfig() => ();
+
+  // Call UseHeadlessConfig() and block on the reply. This will ensure that
+  // the native viewport is later created in headless mode.
+  UseHeadlessConfig() => ();
 };
 
 }