Add chromeless class to the wrapper when in chromeless mode.

This makes it easier for CSS rules to target chromeless mode. This is
useful for xblocks which want to use some CSS tweaks specific to the
chromeless view.

lms/templates/courseware/courseware-chromeless.html

@@ -69,7 +69,7 @@ ${HTML(fragment.foot_html())}
 
 </%block>
 
-<div class="course-wrapper">
+<div class="course-wrapper chromeless">
   <section class="course-content" id="course-content">
       <main id="main" tabindex="-1" aria-label="Content">
         ${HTML(fragment.body_html())}