FR: Add API to refresh IDP access token

Firebase has a straightforward API for auth with Google. I can get and store the AccessToken upon authentication. So it seems that is the way I should implement the auth, the new and improved Firebase way. Also, Firebase provides a nice redirect flow that works on mobile devices.

However, there is a huge problem...

This will get the Access Token.

```
firebase.auth().getRedirectResult().then(function(result) {
if (result.credential) {
// This gives you a Google Access Token. You can use it to access the Google API.
var token = result.credential.accessToken;
// ...
}

// The signed-in user info.
var user = result.user;
})
```
The access token is available and I can use it to read/write to the drive, ...for an hour. Once that token expires, I can no longer do anything. The user is still logged into Firebase, so I can continue to use the Firebase services, but I would need to force the user to sign in again to access the drive. This won't do!

I'm looking for the equivalent of `GoogleUser.reloadAuthResponse()` in Firebase.

How can I do this?
What is the recommended way to build a web app that accesses both Firebase services and G Suite?
Are there any official examples?

My project is Chrome Extension. And auth through gapi is not working with it.