Add script that checks license headers of JavaScript files

The script checks if the license header is the BSD-3 license header that
DevTools is licensed under. It excludes files that have license headers
that reference non-Google/non-Chromium authors or are third_party
bundles that are checked into a non-third_party folder.

Change-Id: I35f78036b48d3d7b7dcb9db1e070691282e27a02
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2016747
Commit-Queue: Tim van der Lippe <tvanderlippe@chromium.org>
Reviewed-by: Liviu Rau <liviurau@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
diff --git a/scripts/javascript_natives/index.js b/scripts/javascript_natives/index.js
index c00369c..2d37b54 100644
--- a/scripts/javascript_natives/index.js
+++ b/scripts/javascript_natives/index.js
@@ -190,7 +190,10 @@
 
   fs.writeFileSync(
       path.join(__dirname, '..', '..', 'front_end', 'javascript_metadata', 'NativeFunctions.js'),
-      `// Generated from ${path.relative(path.join(__dirname, '..', '..'), __filename)}
+      `// Copyright 2020 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.
+// Generated from ${path.relative(path.join(__dirname, '..', '..'), __filename)}
 export const NativeFunctions = ${JSON.stringify(functions)};
 `);
 }