Add missing license header

packages/webview_flutter/webview_flutter_android/test/v4/android_navigation_delegate_test.dart

diff --git a/packages/webview_flutter/webview_flutter_android/test/v4/android_navigation_delegate_test.dart b/packages/webview_flutter/webview_flutter_android/test/v4/android_navigation_delegate_test.dart
index 6257f06..dfde021 100644
--- a/packages/webview_flutter/webview_flutter_android/test/v4/android_navigation_delegate_test.dart
+++ b/packages/webview_flutter/webview_flutter_android/test/v4/android_navigation_delegate_test.dart
@@ -1,3 +1,7 @@
+// Copyright 2013 The Flutter 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 'dart:async';
 
 import 'package:flutter_test/flutter_test.dart';