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';
 
 test(function() {
     var headers = new Headers;
     headers.set('User-Agent', 'Mozilla/5.0');
     headers.set('Accept', 'text/html');
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         assert_equals(text, 'hello');
         var req2 = new Request(req);
         assert_true(req.bodyUsed);
         assert_false(req2.bodyUsed);
         return req2.text();
       }).then(function(text) {
         assert_equals(text, '');
       });
   }, 'Consume and pass');
 
+// 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');
+  }, 'RequestContext of a synthetic Request object');
+
 done();