[Fetch API] Put body members directly on Response/Request

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

 self.addEventListener('fetch', function(event) {
     event.respondWith(new Promise(function(resolve) {
         var headers = [];
         event.request.headers.forEach(function(value, key) {
             headers.push([key, value]);
           });
         if (event.request.url.indexOf('asText') != -1) {
-          event.request.body.asText()
+          event.request.text()
             .then(function(result) {
                 resolve(new Response(JSON.stringify({
                     method: event.request.method,
                     headers: headers,
                     body: result
                   })));
               })
         } else if (event.request.url.indexOf('asBlob') != -1) {
-          event.request.body.asBlob()
+          event.request.blob()
             .then(function(result) {
                 resolve(new Response(JSON.stringify({
                     method: event.request.method,
                     headers: headers,
                     body_size: result.size
                   })));
               })
         } else {
           resolve(new Response('url error:' + event.request.url));
         }
       }));
   });