[gn] Updating args --list to make the comment padding optional

Updating the comment to not be prefixed with three spaces for json
output and removing extraneous output.

Change-Id: Iaeee0809ce3cff050a1d3eec5f34f885677802a8
Reviewed-on: https://chromium-review.googlesource.com/1000392
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Julie Hockett <juliehockett@google.com>
Cr-Original-Commit-Position: refs/heads/master@{#548956}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 12bd7ed6ad7773920fe303d54ef1be9b4798ccb1