Don't send mail for policy_checklist comments.

This changes the policy_checklist review handler to not send mail when
publishing comments. This is so patch authors don't receive nag mail,
but reviewers have the review bot comments for reference.

BUG=None
R=agable@chromium.org

Review URL: https://codereview.chromium.org/23576004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/reviewbot@219800 0039d316-1c4b-4281-b951-d872f2087c98