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

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

Index: mojo/services/native_viewport/public/interfaces/native_viewport.mojom
diff --git a/mojo/services/native_viewport/public/interfaces/native_viewport.mojom b/mojo/services/native_viewport/public/interfaces/native_viewport.mojom
index ab1506e6f923026c2d75aa911e54191e2b2d0fb0..1f41fa6bb6d2f9866f1e6905684267d936dcf79b 100644
--- a/mojo/services/native_viewport/public/interfaces/native_viewport.mojom
+++ b/mojo/services/native_viewport/public/interfaces/native_viewport.mojom
@@ -49,3 +49,10 @@ interface NativeViewport {
 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();
+};