[ServiceWorker] Create a new Blob when body.asBody() is called if the size of body is not set.

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]);
           });
-        event.request.body.asText()
-          .then(function(result) {
-              resolve(new Response(JSON.stringify({
-                  method: event.request.method,
-                  headers: headers,
-                  body: result
-                })));
-            })
+        if (event.request.url.indexOf('asText') != -1) {
+          event.request.body.asText()
+            .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()
+            .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));
+        }
       }));
   });