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.

Reproduire le flux clean-account avec autoUpgradeAnonymousUsers activé, en utilisant le composant Login, LoginProvider, StyledFirebaseAuth et les callbacks uiConfig indiqués. Suivre les callbacks de loginProvider.register et la transition de l’état isSignedIn afin de déterminer pourquoi componentDidUpdate ne se déclenche plus ; le travail est terminé lorsque la cause et le comportement de redirection attendu sont documentés ou vérifiés.

Rédigé par le modèle d'indexation à partir du texte de l'issue.