[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) {
+    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
+                })));
+            })
+      }));
+  });