Docserver: Persist stat cache for versioned file systems

chrome/common/extensions/docs/server2/offline_file_system.py

 # Copyright 2013 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.
 
 from file_system import FileSystem, FileNotFoundError
 from future import Future
 
 
 class OfflineFileSystem(FileSystem):
   '''An offline FileSystem which masquerades as another file system. It throws
   FileNotFound error for all operations, and overrides GetIdentity.
   '''
   def __init__(self, fs):
     self._fs = fs
 
   def Read(self, paths, skip_not_found=False):
     if skip_not_found: return Future(value={})
     def raise_file_not_found():
       raise FileNotFoundError('File system is offline, cannot read %s' % paths)
     return Future(callback=raise_file_not_found)
 
   def Stat(self, path):
     raise FileNotFoundError('File system is offline, cannot read %s' % path)
 
   def GetIdentity(self):
     return self._fs.GetIdentity()
+
+  def GetVersion(self):
+    return self._fs.GetVersion()