gn format: cosmetic changes to AnnotatePreferredMultilineAssignment

No intended functionality change.

R=tfarina@chromium.org
BUG=348474

Review URL: https://codereview.chromium.org/778833002

Cr-Original-Commit-Position: refs/heads/master@{#306705}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: eafa81ac5ab4a6870f6e4297c7beebc684a9c6d5