Implement Request.context for Fetch API

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

Index: LayoutTests/http/tests/fetch/script-tests/request.js
diff --git a/LayoutTests/http/tests/fetch/script-tests/request.js b/LayoutTests/http/tests/fetch/script-tests/request.js
index 9924e4a423e6fc42115fdd8184cdb7eebf4604ee..823f47446e965b5a1a88c5726804ea9a14fbf929 100644
--- a/LayoutTests/http/tests/fetch/script-tests/request.js
+++ b/LayoutTests/http/tests/fetch/script-tests/request.js
@@ -570,4 +570,12 @@ promise_test(function(t) {
       });
   }, '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();