Safari iframe localStorage

This is an attempt to reproduce a Safari iframe localStorage synchronization issue from Safari 9.

Steps

First, make sure you've checked out the changes in the PR, built the SDK, and ran yarn link in both packages/firebase and packages/auth.

1. Install dependencies and link: yarn && cd iframe-app && yarn && yarn link firebase @firebase/auth && cd -
2. Copy your firebase config to iframe-app/.env (or hard code it, if that's easier).

# Sample .env
REACT_APP_API_KEY=xx
REACT_APP_AUTH_DOMAIN=xx
REACT_APP_DATABASE_URL=xx
REACT_APP_PROJECT_ID=xx
REACT_APP_STORAGE_BUCKET=xx
REACT_APP_MESSAGING_SENDER_ID=xx
REACT_APP_APP_ID=1:xx

1. Start the top-level app: HOST=127.0.0.1 yarn start
2. Start the iframe-app: cd iframe-app && HOST=localhost yarn start
3. Open the top-level app in two tabs in Safari
4. In one of the tabs, sign in with a popup by clicking the button
5. Once signed in, observe that localStorage in the iframe in both tabs are in sync. If this is not the case, please reach out to dlarocque@google.com and share what you've observed.