Add a comment to allowConnectingInsecureWebSocket in Settings.in

This was to be included in this change, but I forgot to upload the
latest copy.
https://codereview.chromium.org/246893014/

This comment was suggested by abarth@ in this comment.
https://codereview.chromium.org/246893014/#msg12

BUG=85271

Review URL: https://codereview.chromium.org/254563002

git-svn-id: svn://svn.chromium.org/blink/trunk@172468 bbb929c8-8fbe-4397-9dbb-9b2b20218538