Fix the bug Popover doesn't show when it's only rendered on visible

[Swordsman-Inaction]

The case:

render() {
  return (
    {
      this.state.visible && (
        <Popover
          isVisible={true}
          fromRect={x: 10, y: 10, width: 10, height: 10}>
            <View />
        </Popover>
      )
    }
  )
}

In this case, the Popover won't show since the props never change, and this.setState({contentSize: {}, isAwaitingShow: true}) will never be called in componentWillReceiveProps .

This PR is to fix the bug.