Add top level app element

appengine/swarming/elements/imp/stats/partial-line-chart.html

 <!--
 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed by the Apache v2.0 license that can be
 # found in the LICENSE file.
 -->
 <!--
   This in an HTML Import-able file that contains the definition
   of the following elements:
 
     <partial-line-chart>
@@ skipping 91 matching lines @@
           }
         }
 
         view.setColumns(colsToShow);
         return JSON.parse(view.toDataTable().toJSON());
       },
     });
   })();
   </script>
 </dom-module>