Google sign-in broken on Safari when using Private Mode

### Environment

* Operating System version: macOS High Sierra 10.13.4
* Browser version: Safari 11.1 (Private Mode)
* Firebase SDK version: 5.3.0
* Firebase Product: auth (auth, database, storage, etc)

### The problem
signInWithRedirect and signInWithPopUp both don't work in Safari when using Private Mode.
Firefox and Chrome work fine.

signInWithPopUp says
```The popup has been closed by the user before finalizing the operation.```

signInWithRedirect redirects but then tries to close its window even though it's not a popup and then results in a blank screen.
```Can't close the window since it was not opened by JavaScript```

#### Steps to reproduce:
This was noticed by one of our users and we've confirmed the bug.
You can see it for yourself here (uses signInWithPopUp): https://editor.laska.io/login