[XHR] Fix a XHR-stream test script.

LayoutTests/http/tests/xmlhttprequest/resources/response-stream-orphaned-promise.js

 var global = this;
 if (global.importScripts) {
     // Worker case
     importScripts('/resources/testharness.js');
 }
 
 function fetchBody(url) {
     return new Promise(function(resolve, reject) {
         var xhr = new XMLHttpRequest;
         xhr.responseType = 'stream';
         var visited = false;
         xhr.onreadystatechange = function() {
             if (xhr.readyState !== xhr.LOADING || visited)
                 return;
             visited = true;
-            // Note that all provided urls have empty bodies, so
-            // we don't have to read the data.
             xhr.response.getReader().closed.then(resolve, reject);
         };
         xhr.open('GET', url);
         xhr.send();
     });
 }
 
 promise_test(function() {
+    // slow-empty-response.cgi has an empty body, so we can assume that the
+    // closed promise will be resolved without reading data.
     var url = '/xmlhttprequest/resources/slow-empty-response.cgi';
     return fetchBody(url);
 }, 'check if |closed| gets resolved without stream reference');
 
 promise_test(function() {
+    // alow-failure.cgi will output an error after outputting a few valid
+    // chunks, so we will see a rejection.
     var url = '/xmlhttprequest/resources/slow-failure.cgi';
     return fetchBody(url).then(function() {
         assert_unreached('resolved unexpectedly');
     }, function() {
         // rejected as expected
     });
 }, 'check if |closed| gets rejected without explicit stream reference');
 
 if (global.importScripts) {
     // Worker case
     done();
 }