pre-upload: Check if GN files are non-executable.

BUG=chromium:767517
TEST=manuall verified by presubmit

Change-Id: Ie5dd3d610092aab53ff27e3e0197dd0cb69e619a
Reviewed-on: https://chromium-review.googlesource.com/1404814
Commit-Ready: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Tested-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/pre-upload.py b/pre-upload.py
index 14c5e99..5cd9414 100755
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -1541,6 +1541,8 @@
       '*.css',
       '*.ebuild',
       '*.eclass',
+      '*.gn',
+      '*.gni',
       '*.gyp',
       '*.gypi',
       '*.htm',