Clarify comments for methods added in r178921

Clarify comments for methods added in r178921 as per follow-up review comments.

R=pdr@chromium.org

Review URL: https://codereview.chromium.org/423823005

git-svn-id: svn://svn.chromium.org/blink/trunk@179044 bbb929c8-8fbe-4397-9dbb-9b2b20218538