Implement Request.context for Fetch API

LayoutTests/http/tests/serviceworker/resources/fetch-request-xhr-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 = [];
         for (var header of event.request.headers) {
           headers.push(header);
         }
         event.request.text()
           .then(function(result) {
               resolve(new Response(JSON.stringify({
                   method: event.request.method,
+                  context: event.request.context,
                   mode: event.request.mode,
                   credentials: event.request.credentials,
                   headers: headers,
                   body: result
                 })));
             });
       }));
   });