Remove all unused variables.

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

 # Copyright (C) 2009 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 186 matching lines @@
                 continue
             if path:
                 path += sep
             path += comp
         if comps[-1] == '' and path:
             path += '/'
         path = path.replace(sep + sep, sep)
         return path
 
     def listdir(self, path):
-        root, dirs, files = list(self.walk(path))[0]
+        _, dirs, files = list(self.walk(path))[0]
         return dirs + files
 
     def walk(self, top):
         sep = self.sep
         if not self.isdir(top):
             raise OSError("%s is not a directory" % top)
 
         if not top.endswith(sep):
             top += sep
 
@@ skipping 275 matching lines @@
         return self.data.readline(length)
 
     def __iter__(self):
         return self.data.__iter__()
 
     def next(self):
         return self.data.next()
 
     def seek(self, offset, whence=os.SEEK_SET):
         self.data.seek(offset, whence)