async function getBroadcastPositionUpdates(callback) {
  socket.on('broadcastPosition', async (positionUpdates) => {
    positionUpdates.forEach((positionUpdate) => {
      positionUpdate.position = Position.fromJson(positionUpdate.position);
    });
    await callback(positionUpdates);
  });
}
Listen for PositionUpdates broadcast by the server.
@param {!Function} callback The callback handler that passes the
PositionUpdates to UserDataManager.
@public