Update some comments about variations headers API use.

Based on post-review comments on:
https://codereview.chromium.org/2558913003/

BUG=none
TBR=mattm@chromium.org,donnd@chromium.org,afakhry@chromium.org,noyau@chromium.org

Review-Url: https://codereview.chromium.org/2627033003
Cr-Original-Commit-Position: refs/heads/master@{#443250}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: b2ea4c9c7a315e6da453842076f6ebee657bd371
1 file changed