Use a pinned version of web-platform-tests/manifest.

third_party/WebKit/Tools/Scripts/webkitpy/w3c/deps_updater.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Fetches a copy of the latest state of a W3C test repository and commits.
 
 If this script is given the argument --auto-update, it will also attempt to
 upload a CL, triggery try jobs, and make any changes that are required for
 new failing tests before committing.
 """
@@ skipping 140 matching lines @@
     def _generate_manifest(self, original_repo_path, dest_path):
         """Generates MANIFEST.json for imported tests.
 
         Args:
             original_repo_path: Path to the temporary source WPT repo directory.
             dest_path: Path to the destination WPT directory.
 
         Runs the (newly-updated) manifest command if it's found, and then
         stages the generated MANIFEST.json in the git index, ready to commit.
         """
-        manifest_command = self.fs.join(original_repo_path, 'manifest')
-        if not self.fs.exists(manifest_command):
+        manifest_command = self.finder.path_from_webkit_base('Tools', 'Scripts', 'webkitpy', 'thirdparty', 'wpt', 'manifest')
+        if 'css' in dest_path:
             # Do nothing for csswg-test.
             return
         _log.info('Generating MANIFEST.json')
         self.run([manifest_command, '--work', '--tests-root', dest_path])
         self.run(['git', 'add', self.fs.join(dest_path, 'MANIFEST.json')])
 
     def update(self, dest_dir_name, url, keep_w3c_repos_around, revision):
         """Updates an imported repository.
 
         Args:
@@ skipping 279 matching lines @@
         """Returns a dict mapping source to dest name for layout tests that have been renamed."""
         out = self.check_run(['git', 'diff', 'origin/master', '-M100%', '--diff-filter=R', '--name-status'])
         renamed_tests = {}
         for line in out.splitlines():
             _, source_path, dest_path = line.split()
             source_test = self.finder.layout_test_name(source_path)
             dest_test = self.finder.layout_test_name(dest_path)
             if source_test and dest_test:
                 renamed_tests[source_test] = dest_test
         return renamed_tests