gn format: Better formatting of list-of-single-scope

Fuchsia uses these for test specifications in packages. Blocks are
always formatted multiline, but the list formatting didn't account for
that, so it just turned into a mess when there was only a single element
in the list (see linked bug).

Formatting changes:
https://chromium-review.googlesource.com/c/chromium/src/+/861256

Bug: 800955
Change-Id: I4ab0866d0917071ba94d011c455499a3ba8fb552
Reviewed-on: https://chromium-review.googlesource.com/861282
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#528703}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9b2c266d5882c89c65a1095d9e5d68e435ede1f3