Update comments related to WebP compression format sniffing.

This patch contains only cosmetic changes, in response to some
post-land comments on this previous patch:
https://chromium-review.googlesource.com/c/chromium/src/+/1569691

Bug: 955580
Change-Id: Iffe03cced30f01b3357fc8fc700c2c267cd8677b
TBR: schenney@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1582050
Commit-Queue: Paul Meyer <paulmeyer@chromium.org>
Reviewed-by: Paul Meyer <paulmeyer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653686}
1 file changed