[Fetch API] Implement Request.formData and Response.formData.

third_party/WebKit/LayoutTests/http/tests/fetch/script-tests/response.js

 if (self.importScripts) {
   importScripts('../resources/fetch-test-helpers.js');
   importScripts('/streams/resources/rs-utils.js');
 }
 
 function decode(chunks) {
   var decoder = new TextDecoder();
   var result = '';
   for (var chunk of chunks) {
     result += decoder.decode(chunk, {stream: true});
@@ skipping 263 matching lines @@
 
 promise_test(function(t) {
     return new Response().blob().then(blob => {
         assert_equals(blob.constructor, Blob);
         assert_equals(blob.size, 0);
         assert_equals(blob.type, '');
       });
   }, 'call blob() on null body response');
 
 promise_test(function(t) {
-    return new Response().json().then(unreached_rejection(t), e => {
+    return new Response().formData().then(unreached_fulfillment(t), e => {
+        assert_equals(e.constructor, TypeError);
+      });
+  }, 'call formData() on null body response');
+
+promise_test(function(t) {
+    return new Response().json().then(unreached_fulfillment(t), e => {
         assert_equals(e.constructor, SyntaxError);
       });
   }, 'call json() on null body response');
 
 promise_test(function(t) {
     var res = new Response('hello');
     var body = res.body;
     var clone = res.clone();
     assert_false(res.bodyUsed);
     assert_false(clone.bodyUsed);
@@ skipping 264 matching lines @@
 
 promise_test(t => {
     var controller;
     var stream = new ReadableStream({start: c => controller = c});
     setTimeout(() => controller.enqueue('hello'), 1);
     var response = new Response(stream);
     return promise_rejects(t, TypeError(), response.text());
   }, 'Response constructed stream with a string chunk');
 
 done();