let pht expect PhutilSafeHTMLProducerInterface
Summary: phutil_escape_html can handle these objects, but pht needs to know to call it.
Test Plan: Render a couple of things? I'm not sure how much damage this can do.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Differential Revision: https://secure.phabricator.com/D16244