function onChildComplete() {
    if (--pendingChildren == 0 && callback) {
      callback();
    }
  }
Read the backing storage for these preferences.

You should do this once at initialization time to prime the local cache
of preference values.  The preference manager will monitor the backing
storage for changes, so you should not need to call this more than once.

This function recursively reads storage for all child preference managers as
well.

This function is asynchronous, if you need to read preference values, you
*must* wait for the callback.

@param {function()=} callback Optional function to invoke when the read
    has completed.