web_dev_style: actually restore sys.path in various PRESUBMITs

ui/webui/resources/PRESUBMIT.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.
 
 def PostUploadHook(cl, change, output_api):
   return output_api.EnsureCQIncludeTrybotsAreAdded(
     cl,
     [
       'master.tryserver.chromium.linux:closure_compilation',
     ],
@@ skipping 43 matching lines @@
 <cr-dialog>#closeText (http://bit.ly/2eLEsqh).""")]
 
 
 def _CommonChecks(input_api, output_api):
   results = []
   results += _CheckForTranslations(input_api, output_api)
   results += input_api.canned_checks.CheckPatchFormatted(input_api, output_api,
                                                          check_js=True)
   try:
     import sys
-    old_sys_path = sys.path
+    old_sys_path = sys.path[:]
     cwd = input_api.PresubmitLocalPath()
     sys.path += [input_api.os_path.join(cwd, '..', '..', '..', 'tools')]
     from web_dev_style import presubmit_support
     BLACKLIST = ['ui/webui/resources/js/analytics.js',
                  'ui/webui/resources/js/jstemplate_compiled.js']
     file_filter = lambda f: f.LocalPath() not in BLACKLIST
     results += presubmit_support.CheckStyle(input_api, output_api, file_filter)
   finally:
     sys.path = old_sys_path
   return results