Don't set feed width on chromeless page

Summary: Otherwise browser displays horizontal scrollbar at http://phabricator.org/.

Test Plan:
Add this rule in Firebug and display http://phabricator.org/.

/feed/

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2173