Link anonymous user with logged in user, possible bug. componentDidMount not called anymore.

Hello,
first of all Happy Easter.

I think there is a bug when using `autoUpgradeAnonymousUsers: true` in `uiConfig`.

I will paste my code and I'll explain how to reproduce.

*initialization code:*
```
const uiConfig = {
signInFlow: 'popup',
c
signInOptions: [
firebase.auth.GoogleAuthProvider.PROVIDER_ID,
],
callbacks: {
signInSuccessWithAuthResult: () => {
console.log('signInSuccessWithAuthResult')
return false
},
signInFailure: (error: any) => {
console.log('sign')
if (error.code != 'firebaseui/anonymous-upgrade-merge-conflict') {
return Promise.resolve();
}
var cred = error.credential;
return firebase.auth().signInWithCredential(cred);
}

},
};

const loginProvider = new FirebaseLoginProvider(
firebaseApp, uiConfig
);

loginProvider.register((user) => {
if (user == null) {
console.log('null')
firebaseApp.auth().signInAnonymously().catch((error) => {
console.log(error);
})
} else {
console.log("Anony " + user.isAnonymous)
}
})
```

And this is my react component:
```
import 'firebase/auth';
import './firebase-global.css';

import * as React from 'react';

import { RouteComponentProps, withRouter } from "react-router-dom";

import LoginProvider from './LoginProvider';
import StyledFirebaseAuth from 'react-firebaseui/StyledFirebaseAuth';

interface Props {
};

interface State {
isSignedIn: boolean
}

interface HomeProps extends RouteComponentProps {
loginProvider: LoginProvider
}

class Login extends React.Component {

state = {
isSignedIn: false
}

constructor(props: HomeProps) {
super(props);
}

componentDidMount() {
console.log('componentDidMount')
this.props.loginProvider.register((user) => {
if (user != null && !user.isAnonymous) {
this.setState({ isSignedIn: true } as State);
} else {
this.setState({ isSignedIn: false } as State);
}
})
}

componentDidUpdate() {
console.log('componentDidUpdate')
this.goBackWhenLoggedIn();
}

componentWillUnmount() {
console.log('componentWillUnmount')
this.props.loginProvider.unregister();
}

render() {
const signedIn = this.state.isSignedIn

return (

);
}

private goBackWhenLoggedIn() {
if (this.state.isSignedIn) {
// this.props.loginProvider.promoteAccount()
this.props.history.goBack()
}
}

}
export default withRouter(Login);
```

After login, before adding the option `autoUpgradeAnonymousUsers: true` for which I also had to add
```
signInFailure: (error: any) => {
console.log('sign')
if (error.code != 'firebaseui/anonymous-upgrade-merge-conflict') {
return Promise.resolve();
}
var cred = error.credential;
return firebase.auth().signInWithCredential(cred);
}
```
based on documentation from *Handling anonymous user upgrade merge conflicts* paragraph on https://firebase.google.com/docs/auth/web/firebaseui

I don't get redirected successfully WHEN I have a clean situation for my user meaning I delete my anonymous user and my logged in user from firebase console (Manage users).

Before, after logging in on Google, `componentDidUpdate` was called and I could succesfully be redirected to the last page using react router. Now I can't anymore.

What I did to solve the issue is to use `window.history.back();` inside `signInSuccessWithAuthResult` function.

Am I doing something wrong with the configuration? I can't understand why `componentDidUpdate` is not fired anymore.

Reproduce the clean-account flow with autoUpgradeAnonymousUsers enabled, using the Login component, LoginProvider, StyledFirebaseAuth, and the shown uiConfig callbacks. Trace loginProvider.register callbacks and the isSignedIn state transition to determine why componentDidUpdate no longer triggers; done means the cause and expected redirect behavior are documented or verified.

Written by the indexing model from the issue text.