tools/gn: Make it 'gn check' clean.

This was checked with the following command line:

$ out/Debug/gn check out_gn/Debug //tools/gn/*

And cleaned until it reported "Header dependency check OK".

BUG=367595,376000
TEST=see above
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/806873003

Cr-Original-Commit-Position: refs/heads/master@{#309371}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: b8e602bfd91331d0d7976ce6e45f846c6d1a8433
1 file changed
tree: 9dbe982703947989e4dfcf83f64d0b67c2f9a5be
  1. tools/