Add flag_content.sid to Views flag_views_data() hook

The background here is that a colleague has set up a View with VBO functionality to act as a moderation queue for nodes that have been flagged for 'abuse' by end users. The flag in question allows for anonymous flagging.

The current VBO callbacks use the Flag module flag('unflag', 'flag_name', $content_id, $account) function to remove the flag from the node ('allow' action removes the flag only, 'remove' action removes the flag and unpublishes the node). This works if the flag in question is from an authenticated user but it does not work when the flag was done by an anonymous user.

The flag method of the flag object expects that for a $flag->flag() method call that if it's an anonymous user that that user is the current user in context. However if you are processing nodes with flags in an admin context such as via VBO there is no current anonymous user context

In flag.inc:

    // Find out which user id to use.
    $uid = $this->global ? 0 : $account->uid;

    // Find out which session id to use.
    if ($this->global) {
      $sid = 0;
    }
    else {
      $sid = flag_get_sid($uid, TRUE);
      // Anonymous users must always have a session id.
      if ($sid == 0 && $account->uid == 0) {
        return FALSE;
      }
    }

If one has the content_id, uid, and sid of a flag_content record then it's possible to directly call the private method $flag->_unflag() to remove the flag thus bypassing the check above for current user context for anonymous flags. It's something of a workaround but it gets the job done absent an alternative form of removing a flag record programmitically.

Since for anonymous user flags the only distinction from one record in flag_content to the next is the flag_content.sid field, I'd like to have that field available as a Views field. Once the sid field is able to be used in a View then for the case of our VBO action callback we can use the sid to invoke the private _unflag() method.

The attached patch adds sid to the hook_views_data implementation.