function shouldUpdateReactComponent(prevDescriptor, nextDescriptor) {
  if (prevDescriptor && nextDescriptor &&
      prevDescriptor.type === nextDescriptor.type && (
        (prevDescriptor.props && prevDescriptor.props.key) ===
        (nextDescriptor.props && nextDescriptor.props.key)
      ) && prevDescriptor._owner === nextDescriptor._owner) {
    return true;
  }
  return false;
}
Given a `prevDescriptor` and `nextDescriptor`, determines if the existing
instance should be updated as opposed to being destroyed or replaced by a new
instance. Both arguments are descriptors. This ensures that this logic can
operate on stateless trees without any backing instance.

@param {?object} prevDescriptor
@param {?object} nextDescriptor
@return {boolean} True if the existing instance should be updated.
@protected