Update GN reference.md.

https://chromium-review.googlesource.com/c/chromium/src/+/642717 added
a new switch to `gn args`. Update the reference.md with the new docs.

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

Change-Id: I06be347e72cab84d2b3bf4c887c59489da9f30f2
Reviewed-on: https://chromium-review.googlesource.com/644487
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Mike Bjorge <mbjorge@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#498646}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f751b31e6323c40033a48c4b9d1fef009cad2ced
1 file changed
tree: 99bdae1905570a67db3f752433eb03a25340f9ef
  1. tools/