Implement Request.context for Fetch API

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

 var requests = [];
 var port = undefined;
 
 self.onmessage = function(e) {
   var message = e.data;
   if ('port' in message) {
     port = message.port;
     port.postMessage({ready: true});
   }
 };
 
 self.addEventListener('fetch', function(event) {
     var url = event.request.url;
     if (url.indexOf('dummy?test') == -1) {
       return;
     }
     port.postMessage({
         url: url,
+        context: event.request.context,
+        context_clone: event.request.clone().context,
+        context_new: (new Request(event.request)).context,
         mode: event.request.mode,
         credentials: event.request.credentials
       });
     event.respondWith(Promise.reject());
   });