Create a common directory of widgets and css.

appengine/chromium_rietveld/new_static/common/cr-collapsible.html

 <!-- Copyright (c) 2015 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file. -->
 
 <!--
     Based on <polymer-ui-collapsible>, this widget expects a child element
     that has class name "cr-collapsible-header" and will expand or collapse of
     the rest of the children when the header is tapped using animation.
 
     ex.
@@ skipping 101 matching lines @@
                     return;
                 this.updateSize(this.computeBodySize());
                 this.async(function() {
                     this.computeBodySize();
                     this.updateSize("0", true);
                 });
             },
         });
   </script>
 </polymer-element>