HTMLHiddenField: Support CodexHTMLForm
HTMLHiddenField does some black magic by outputting nothing and then
adding a hidden field to the form somewhere else. This black magic
wasn't updated for CodexHTMLForm, so it broke on forms that use Codex
(specifically, the login form, where CAPTCHAs are added using
HTMLHiddenField).
Bug: T360717
Change-Id: I2faec05d809b926b2ec2dedaceb7c74a445e06b2