Add missing source to gn bootstrap.

This command is subject to bitrot, and this seems to be today's
instance.

BUG=510240
TEST=tools/gn/bootstrap/bootstrap.py

R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1235233005

Cr-Original-Commit-Position: refs/heads/master@{#338766}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d673f55c138f6b263375c096c43ecbb929246021
1 file changed
tree: 0742b323a07335a639ecadba3f9da50342c327ff
  1. tools/