Add a headless configuration to Mojo's native viewport service

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

Index: mojo/services/public/interfaces/native_viewport/native_viewport.mojom
diff --git a/mojo/services/public/interfaces/native_viewport/native_viewport.mojom b/mojo/services/public/interfaces/native_viewport/native_viewport.mojom
index 6c9fd199a93070aa8de2fa734f8d717eba0c763e..61afb1aa7eafa2273fabfd5a7101a7cd9b67958c 100644
--- a/mojo/services/public/interfaces/native_viewport/native_viewport.mojom
+++ b/mojo/services/public/interfaces/native_viewport/native_viewport.mojom
@@ -26,11 +26,16 @@ interface NativeViewportClient {
   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() => ();
 };
 
 }