Stores stat entry in the flow's state.

def StoreStat(self, responses):
    """Stores stat entry in the flow's state."""
    index = responses.request_data["index"]
    if not responses.success:
      self.Log("Failed to stat file: %s", responses.status)
      # Report failure.
      self._FileFetchFailed(index, responses.request_data["request_name"])
      return

    tracker = self.state.pending_hashes[index]
    tracker["stat_entry"] = responses.First()