Can't import the sdk inside a service worker

**Describe the bug**
If I simply import the SDK in a service worker, I get an error
```
import * as sdk from 'matrix-js-sdk';
```
**Error**
> Uncaught ReferenceError: XMLHttpRequest is not defined

**Expected behavior**
Importing the SDK should work

**Additional context**
I'm trying to build a chrome extension, and so have to run the SDK in a background script or a service worker.

Start by reproducing the `import * as sdk from 'matrix-js-sdk'` entry point in a Chrome service worker and trace where the `XMLHttpRequest is not defined` error occurs. Done means the SDK can be imported in that environment without requiring `XMLHttpRequest`; add or update a focused regression test if the repository has coverage for this entry point.

Written by the indexing model from the issue text.