Adds the field paths descended from node to field_mask.

def _AddFieldPaths(node, prefix, field_mask):
  """Adds the field paths descended from node to field_mask."""
  if not node:
    field_mask.paths.append(prefix)
    return
  for name in sorted(node):
    if prefix:
      child_path = prefix + '.' + name
    else:
      child_path = name
    _AddFieldPaths(node[name], child_path, field_mask)