bind-gen: Add a file header comment

Adds a caution of auto-generated files.

Bug: 839389
Change-Id: I212dd2eb8153669f964f7b1a1de3d7ad4c9f8bf5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2358931Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798607}

third_party/blink/renderer/bindings/scripts/bind_gen/codegen_utils.py

@@ -20,7 +20,10 @@ def make_copyright_header():
     return LiteralNode("""\
 // Copyright 2019 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.\
+// found in the LICENSE file.
+
+// DO NOT EDIT: This file is auto-generated by
+// //third_party/blink/renderer/bindings/scripts/generate_bindings.py\
 """)