gn format: fix not indenting multiline continued correctly

061.gn is how it was formatted before, which is misleading.

R=brettw@chromium.org
BUG=348474

Review URL: https://codereview.chromium.org/774753004

Cr-Original-Commit-Position: refs/heads/master@{#306566}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d83fc265629d2a24078132b260c76192f3cc68f1