Fix copyright headers in GN files

Fixes a few more copyright headers that were missed earlier because
PanProjectChecks() doesn't consider GN files as source files.

Bug: pdfium:1884
Change-Id: I0d0f47750d1760bfea1f9aa8831f87accf2725c3
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/100630
Auto-Submit: K. Moon <kmoon@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: K. Moon <kmoon@chromium.org>
diff --git a/skia/BUILD.gn b/skia/BUILD.gn
index dd9528f..6eae4bb 100644
--- a/skia/BUILD.gn
+++ b/skia/BUILD.gn
@@ -1,4 +1,4 @@
-# Copyright (c) 2013 The Chromium Authors. All rights reserved.
+# Copyright 2013 The PDFium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.