webkit_python_tests: always run from the checkout root

testing/scripts/webkit_python_tests.py

 #!/usr/bin/env python
 # Copyright 2015 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 json
 import os
 import sys
 
 
 import common
 
 
 def main_run(args):
   with common.temporary_file() as tempfile_path:
     rc = common.run_command([
         sys.executable,
         os.path.join(common.SRC_DIR, 'third_party', 'WebKit',
                      'Tools', 'Scripts', 'test-webkitpy'),
         '--write-full-results-to', tempfile_path,
-    ])
+    ], cwd=args.paths['checkout'])
 
     with open(tempfile_path) as f:
       results = json.load(f)
 
   parsed_results = common.parse_common_test_results(results)
   failures = parsed_results['unexpected_failures']
 
   json.dump({
       'valid': bool(rc <= common.MAX_FAILURES_EXIT_STATUS and
                    ((rc == 0) or failures)),
       'failures': failures.keys(),
   }, args.output)
 
   return rc
 
 
 def main_compile_targets(args):
   json.dump([], args.output)
 
 
 if __name__ == '__main__':
   funcs = {
     'run': main_run,
     'compile_targets': main_compile_targets,
   }
   sys.exit(common.run_script(sys.argv[1:], funcs))