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

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

Index: LayoutTests/http/tests/local/serviceworker/resources/fetch-request-body-file-worker.js
diff --git a/LayoutTests/http/tests/serviceworker/resources/fetch-request-xhr-worker.js b/LayoutTests/http/tests/local/serviceworker/resources/fetch-request-body-file-worker.js
similarity index 62%
copy from LayoutTests/http/tests/serviceworker/resources/fetch-request-xhr-worker.js
copy to LayoutTests/http/tests/local/serviceworker/resources/fetch-request-body-file-worker.js
index b58233da57b84a83b2a256c1b1d611a887f7095f..83d7445dc815185fbabcdc0e35c45e84dafaa7d5 100644
--- a/LayoutTests/http/tests/serviceworker/resources/fetch-request-xhr-worker.js
+++ b/LayoutTests/http/tests/local/serviceworker/resources/fetch-request-body-file-worker.js
@@ -1,21 +1,9 @@
 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({