GN comment spelling fix.

This addresses a review comment from https://codereview.chromium.org/1373903002/ that I forgot to fix there.

TBR=andybons@chromium.org

Review URL: https://codereview.chromium.org/1370293002

Cr-Original-Commit-Position: refs/heads/master@{#351145}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 70e884bceb30768f4161aa65275c5d82f0e62e77