tools/gn: update reference.md doc

This patch was generated by the following command line:

$ gn help --markdown all > tools/gn/docs/reference.md

BUG=None
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/1500923003

Cr-Original-Commit-Position: refs/heads/master@{#363298}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 0cbfc07f673b7445fa43b2f115f1c79c07a6e1ca
1 file changed
tree: 34ebca5cd8e1a46ede18a81aa44abefa5d319a0c
  1. tools/