[ServiceWorker] Add test for FetchEvent's body with a local file.

LayoutTests/http/tests/local/serviceworker/resources/fetch-request-body-file-worker.js

 self.addEventListener('fetch', function(event) {
-    var url = event.request.url;
-    if (url.indexOf('dummy?test') == -1) {
-      return;
-    }
     event.respondWith(new Promise(function(resolve) {
         var headers = [];
         event.request.headers.forEach(function(value, key) {
             headers.push([key, value]);
           });
-        if (!event.request.body) {
-          resolve(new Response(JSON.stringify({
-              method: event.request.method,
-              headers: headers,
-              body: null
-            })));
-          return;
-        }
         event.request.body.asText()
           .then(function(result) {
               resolve(new Response(JSON.stringify({
                   method: event.request.method,
                   headers: headers,
                   body: result
                 })));
             })
       }));
   });