function onChildStorage() {
    if (--pendingChildren == 0 && callback) {
      callback();
    }
  }
Synchronize a list of child PreferenceManagers instances with the current
list stored in prefs.

This will instantiate any missing managers and read current preference values
from storage.  Any active managers that no longer appear in preferences will
be deleted.

@param {string} listName The child list to synchronize.
@param {function()=} callback Function to invoke when the sync finishes.