Allow native_viewport to create new native windows on demand on Android.

mojo/services/native_viewport/public/interfaces/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.
 
 [DartPackage="mojo_services"]
 module mojo;
 
 import "geometry/public/interfaces/geometry.mojom";
 import "gpu/public/interfaces/context_provider.mojom";
 import "input_events/public/interfaces/input_events.mojom";
@@ skipping 31 matching lines @@
   // method. Call RequestMetrics() to receive updates when the viewport metrics
   // change. The reply will be sent when the viewport metrics are different from
   // the values last sent, so to receive continuous updates call this method
   // again after receiving the callback.
   RequestMetrics() => (ViewportMetrics metrics);
 };
 
 interface NativeViewportEventDispatcher {
   OnEvent(Event event) => ();
 };
+
+// Service to be used by the Shell to notify NativeViewport of major changes.
+interface NativeViewportShellService {
+  // A new native surface (e.g.: a new activity instance on Android) is
+  // available and can be used for rendering.
+  NewNativeSurfaceAvailable();
+};