UseCounter: Introduce UseCounter::Observer for layout tests

third_party/WebKit/LayoutTests/fast/workers/resources/shared-worker-usecounter.js

 onconnect = e => {
   var port = e.ports[0];
   port.onmessage = (e) => {
     if (e.data.type == 'COUNT_FEATURE')
       internals.countFeature(e.data.feature);
     else if (e.data.type == 'COUNT_DEPRECATION')
       internals.countDeprecation(e.data.feature);
-    port.postMessage('COUNTED');
   };
   port.postMessage('CONNECTED');
 }