Fetch: Make Headers' constructor match the current spec IDL.

third_party/WebKit/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');
@@ skipping 664 matching lines @@
       .then(function() {
           t.done();
         })
       .catch(unreached_rejection(t));
     assert_true(request.bodyUsed,
                 'bodyUsed must be true after calling text()');
   }, 'Request body test');
 
 test(function() {
     // https://fetch.spec.whatwg.org/#dom-request
-    // Step 20:
+    // Step 32:
     // Fill r's Headers object with headers. Rethrow any exceptions.
     INVALID_HEADER_NAMES.forEach(function(name) {
         assert_throws(
           {name: 'TypeError'},
           function() {
             var obj = {};
             obj[name] = 'a';
             new Request('http://localhost/', {headers: obj});
           },
           'new Request with headers with an invalid name (' + name +
@@ skipping 117 matching lines @@
                            credentials: 'include',
                            body: 'this is a body'});
 
     return req.text()
         .then(t => {
            assert_equals(t, 'this is a body');
         });
   }, 'Credentials and body can both be set.');
 
 done();