Fix all "dangerous-default-value" pylint warnings.

third_party/WebKit/Tools/Scripts/webkitpy/common/system/filesystem.py

 # Copyright (C) 2010 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 61 matching lines @@
 
     def copyfile(self, source, destination):
         shutil.copyfile(source, destination)
 
     def dirname(self, path):
         return os.path.dirname(path)
 
     def exists(self, path):
         return os.path.exists(path)
 
-    def files_under(self, path, dirs_to_skip=[], file_filter=None):
+    def files_under(self, path, dirs_to_skip=None, file_filter=None):
         """Return the list of all files under the given path in topdown order.
 
         Args:
             dirs_to_skip: a list of directories to skip over during the
                 traversal (e.g., .svn, resources, etc.)
             file_filter: if not None, the filter will be invoked
                 with the filesystem object and the dirname and basename of
                 each file found. The file is included in the result if the
                 callback returns True.
         """
+        dirs_to_skip = dirs_to_skip or []
+
         def filter_all(fs, dirpath, basename):
             return True
 
         file_filter = file_filter or filter_all
         files = []
         if self.isfile(path):
             if file_filter(self, self.dirname(path), self.basename(path)):
                 files.append(path)
             return files
 
@@ skipping 171 matching lines @@
     def split(self, path):
         """Return (dirname, basename + '.' + ext)"""
         return os.path.split(path)
 
     def splitext(self, path):
         """Return (dirname + os.sep + basename, '.' + ext)"""
         return os.path.splitext(path)
 
     def make_executable(self, file_path):
         os.chmod(file_path, stat.S_IRUSR | stat.S_IWUSR | stat.S_IXUSR | stat.S_IRGRP | stat.S_IXGRP)