Added TodoMVC startup benchmarks.

samples/third_party/todomvc_performance/web/elements/td_item.css

 /* base.css overrides */
 @host {
   .editing {
     border-bottom: none;
     padding: 0;
   }
 }
 
 button {
   margin: 0;
@@ skipping 141 matching lines @@
       ShadowDOM Polyfill work around for webkit/blink bug
       https://code.google.com/p/chromium/issues/detail?id=251510
     */
     background-color: transparent;
   }
 
   .toggle {
     height: 40px;
   }
 }