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>
1 file changed