So what you're attempting to install here is a chrome app[1]. It uses the Chrome Extension/App APIs to create "desktop app" experience. Hangouts as a part of gmail/google+ I believe is supported on Google supported browsers[2].
[1]: https://chrome.google.com/webstore/detail/hangouts/nckgahada... [2]: https://support.google.com/a/answer/33864?hl=en