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

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

 # Copyright 2016 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.
 
 import unittest
 
 from webkitpy.common.host_mock import MockHost
 from webkitpy.common.system.executive_mock import MockExecutive
 from webkitpy.w3c.deps_updater import DepsUpdater
 
@@ skipping 86 matching lines @@
         updater = DepsUpdater(host)
         updater._generate_manifest(
             '/mock-checkout/third_party/WebKit/css',
             '/mock-checkout/third_party/WebKit/LayoutTests/imported/csswg-test')
         self.assertEqual(host.executive.calls, [])
 
     def test_generate_manifest_successful_run(self):
         # This test doesn't test any aspect of the real manifest script, it just
         # asserts that DepsUpdater._generate_manifest would invoke the script.
         host = MockHost()
-        host.filesystem.files = {
-            '/mock-checkout/third_party/WebKit/wpt/manifest': 'dummy content'
-        }
         updater = DepsUpdater(host)
         updater._generate_manifest(
             '/mock-checkout/third_party/WebKit/wpt',
             '/mock-checkout/third_party/WebKit/LayoutTests/imported/wpt')
         self.assertEqual(
             host.executive.calls,
             [
                 [
-                    '/mock-checkout/third_party/WebKit/wpt/manifest',
+                    '/mock-checkout/third_party/WebKit/Tools/Scripts/webkitpy/thirdparty/wpt/manifest',
                     '--work',
                     '--tests-root',
                     '/mock-checkout/third_party/WebKit/LayoutTests/imported/wpt'
                 ],
                 [
                     'git',
                     'add',
                     '/mock-checkout/third_party/WebKit/LayoutTests/imported/wpt/MANIFEST.json'
                 ]
             ])