Delay generation of User-Agent header to URLRequestHttpJob and accept custom User-Agent from XHR/Fe…

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');
     headers.set('X-Fetch-Test', 'request test field');
 
     var request = new Request(URL, {method: 'GET', headers: headers});
 
     assert_equals(request.url, URL, 'Request.url should match');
     assert_equals(request.method, 'GET', 'Request.method should match');
     assert_equals(request.referrer, 'about:client',
                   'Request.referrer should be about:client');
     assert_equals(request.referrerPolicy, '',
                   'Request.referrerPolicy should be empty');
     assert_true(request.headers instanceof Headers,
                 'Request.headers should be Headers');
 
-    // 'User-Agent' is a forbidden header.
-    assert_equals(size(request.headers), 2,
+    assert_equals(size(request.headers), 3,
                   'Request.headers size should match');
     // Note: detailed behavioral tests for Headers are in another test,
     // http/tests/fetch/*/headers.html.
 
     request.url = 'http://localhost/';
     assert_equals(request.url, 'https://www.example.com/test.html',
                   'Request.url should be readonly');
 
     // Unmatched lead surrogate.
     request = new Request('http://localhost/\uD800');
@@ skipping 777 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();