iframe tag missing allow-same-origin, prevents loading SDK in iframe

### [REQUIRED] Describe your environment

* Operating System version: Windows 10
* Browser version: Edge 90.0.818.51
* Firebase SDK version: 8.2.1
* Firebase Product: database

### [REQUIRED] Describe the problem

iframing a cross-domain page that invokes the firebase js sdk seems to result in an error.

```
BrowserPollConnection.ts:503 Sandbox access violation: Blocked a frame at "https://overlays.rtirl.com" from accessing a frame at "null". Both frames are sandboxed and lack the "allow-same-origin" flag.
BrowserPollConnection.ts:518 Uncaught DOMException: Blocked a frame with origin "null" from accessing a cross-origin frame.
at Function.ji.createIFrame_ (https://overlays.rtirl.com/__/firebase/8.2.9/firebase-database.js:1:110223)
at new ji (https://overlays.rtirl.com/__/firebase/8.2.9/firebase-database.js:1:112331)
at n (https://overlays.rtirl.com/__/firebase/8.2.9/firebase-database.js:1:105983)
at Qi.open (https://overlays.rtirl.com/__/firebase/8.2.9/firebase-database.js:1:107180)
at https://overlays.rtirl.com/__/firebase/8.2.9/firebase-database.js:1:117875
```

The relevant failure line is here: https://github.com/firebase/firebase-js-sdk/blob/master/packages/database/src/realtime/BrowserPollConnection.ts#L500

#### Steps to reproduce:

Domain foo.com:

```

```

Domain bar.com:

```

firebase.initializeApp(...);
firebase.database()...

```

Should result in the above exception.

If I understand same-origin policies correctly, I believe the sdk just needs to add the `sandbox="allow-same-origin"` attr to the created iframe element?