Fix MockFileSystem.walk and add test.

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 187 matching lines @@
         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]
         return dirs + files
 
     def walk(self, top):
+        FILE_SYSTEM_TUPLES = []

[qyearsley, 2016/06/01 23:08:43]

Non-global variable names use lowercase with underscore, e.g.
file_system_tuples.

         sep = self.sep
         if not self.isdir(top):
             raise OSError("%s is not a directory" % top)
 
         if not top.endswith(sep):
             top += sep
 
         dirs = []
         files = []
         for f in self.files:
             if self.exists(f) and f.startswith(top):
                 remaining = f[len(top):]
                 if sep in remaining:
                     dir = remaining[:remaining.index(sep)]
                     if not dir in dirs:
                         dirs.append(dir)
                 else:
                     files.append(remaining)
-        return [(top[:-1], dirs, files)]
+        FILE_SYSTEM_TUPLES = [(top[:-1], dirs, files)]
+        for dir in dirs:
+            dir = top + dir
+            subdir = self.walk(dir)

[qyearsley, 2016/06/01 23:08:43]

This variable name could probably be improved; is it a tuple or a list of
tuples? The name `subdir` suggests that it may be a string.

+            FILE_SYSTEM_TUPLES += subdir
+        return FILE_SYSTEM_TUPLES
 
     def mtime(self, path):
         if self.exists(path):
             return 0
         self._raise_not_found(path)
 
     def _mktemp(self, suffix='', prefix='tmp', dir=None, **kwargs):
         if dir is None:
             dir = self.sep + '__im_tmp'
         curno = self.current_tmpno
@@ skipping 248 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)