Unify multiple sequential_promise_test definitions.

LayoutTests/http/tests/fetch/resources/fetch-test-helpers.js

 if (self.importScripts) {
   importScripts('/resources/testharness.js');
+  importScripts('/resources/testharness-helpers.js');
   importScripts('/serviceworker/resources/test-helpers.js');
   importScripts('../resources/fetch-test-options.js');
 }
 
 function getContentType(headers) {
   var content_type = '';
   for (var header of headers) {
     if (header[0] == 'content-type')
       content_type = header[1];
   }
   return content_type;
 }
 
-
-(function () {
-  var sequential_promise = Promise.resolve();
-
-  // |sequential_test| is a dummy test to prevent tests from terminating early.
-  // Without this, the number of pending tests recognized by testharness can
-  // be zero, which leads to early unexpected test termination.
-  var sequential_test = undefined;
-
-  // sequential_promise_test() is similar to promise_test(), but multiple tests
-  // are executed one by one sequentially.
-  // Also call sequential_promise_test_done() after all
-  // sequential_promise_test() are called.
-  function sequential_promise_test(func, name) {
-    if (sequential_test === undefined) {
-      sequential_test = async_test('Sequential');
-    }
-    sequential_promise = sequential_promise
-      .then(function() {
-          var promise = undefined;
-          promise_test(function(t) {
-              promise = func(t);
-              return promise;
-            }, name);
-          return promise;
-        })
-      .catch(function(e) {
-          // The error was already reported. Continue to the next test.
-        });
-  }
-
-  function sequential_promise_test_done() {
-    if (sequential_test !== undefined) {
-      sequential_promise.then(function() {sequential_test.done();});
-    }
-  }
-
-  self.sequential_promise_test = sequential_promise_test;
-  self.sequential_promise_test_done = sequential_promise_test_done;
-})();
-
 // token [RFC 2616]
 // "token          = 1*<any CHAR except CTLs or separators>"
 // All octets are tested except for those >= 0x80.
 var INVALID_TOKENS = [
   '',
   // CTL
   '\x00', '\x01', '\x02', '\x03', '\x04', '\x05', '\x06', '\x07',
   '\x08', '\x09', '\x0a', '\x0b', '\x0c', '\x0d', '\x0e', '\x0f',
   '\x10', '\x11', '\x12', '\x13', '\x14', '\x15', '\x16', '\x17',
   '\x18', '\x19', '\x1a', '\x1b', '\x1c', '\x1d', '\x1e', '\x1f', '\x7f',
@@ skipping 182 matching lines @@
                 function() {
                   if (mode !== 'same-origin') {
                     assert_unreached(
                       'Test 5: Cross-origin HTTPS request must success: ' +
                       'mode = ' + mode);
                   }
                 });
           });
     }, 'Block fetch() as mixed content (' + mode + ')');
 }