Docserver: Persist stat cache for versioned file systems

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

Index: chrome/common/extensions/docs/server2/mock_file_system.py
diff --git a/chrome/common/extensions/docs/server2/mock_file_system.py b/chrome/common/extensions/docs/server2/mock_file_system.py
index 76942f809da1f83599927a8383dcae6096bc4795..2fe1a511c91e0c76d190cb9fdb67a4bb9facfa24 100644
--- a/chrome/common/extensions/docs/server2/mock_file_system.py
+++ b/chrome/common/extensions/docs/server2/mock_file_system.py
@@ -86,10 +86,10 @@ class MockFileSystem(FileSystem):
     return stat
 
   def GetCommitID(self):
-    return Future(value=self._stat_tracker.GetVersion(''))
+    return Future(value=str(self._stat_tracker.GetVersion('')))
 
   def GetPreviousCommitID(self):
-    return Future(value=self._stat_tracker.GetVersion('') - 1)
+    return Future(value=str(self._stat_tracker.GetVersion('') - 1))
 
   def GetIdentity(self):
     return self._file_system.GetIdentity()