<iframe :src="'https://player.twitch.tv/?channel=' + currentStreamer.user_name" frameborder="0" allowfullscreen="true" scrolling="no" width="100%"></iframe>
In the HTML becomes
<iframe :src="'https://player.twitch.tv/?channel=' + currentStreamer.user_name + '&parent=localhost" frameborder="0" allowfullscreen="true" scrolling="no" width="100%"></iframe>