Implement Request.context for Fetch API

LayoutTests/http/tests/fetch/script-tests/request.js

 if (self.importScripts) {
   importScripts('../resources/fetch-test-helpers.js');
 }
 
 var URL = 'https://www.example.com/test.html';
 
 function size(headers) {
   var count = 0;
   for (var header of headers) {
     ++count;
@@ skipping 697 matching lines @@
                            body: new Blob([''], {type: 'Text/Plain'}),
                            headers: [['Content-Type', 'Text/Html']]});
     req.body.cancel();
     return req.blob()
       .then(function(blob) {
           assert_equals(blob.type, 'text/html');
           assert_equals(req.headers.get('Content-Type'), 'Text/Html');
         });
   }, 'Extract a MIME type (2)');
 
+// Tests for requests context.
+test(function() {
+    var request = new Request('http://localhost/', {method: 'POST', body: ''});
+    assert_equals(request.context, '',
+                  'Request.context should be empty string for synthetic Request object');

[hiroshige, 2015/04/09 17:43:21]

nit: please wrap to 80 columns, e.g.
                  'Request.context should be empty string ' +
                  'for synthetic Request object');
(https://www.chromium.org/blink/serviceworker/testing#TOC-Layout-Tests-Style)

[shiva.jm, 2015/04/10 09:27:10]

Done.

+  }, 'RequestContext of a synthetic Request object');
+
 done();