Writes a new client crash record.

def WriteClientCrashInfo(self, client_id, crash_info):
    """Writes a new client crash record."""
    if client_id not in self.metadatas:
      raise db.UnknownClientError(client_id)

    ts = rdfvalue.RDFDatetime.Now()
    self.metadatas[client_id]["last_crash_timestamp"] = ts
    history = self.crash_history.setdefault(client_id, {})
    history[ts] = crash_info.SerializeToString()