TypeError: null is not an object (evaluating 'performance.timing.navigationStart') in web worker!

# Package + Version

- [x] `@sentry/browser`
- [ ] `@sentry/node`
- [ ] `raven-js`
- [ ] `raven-node` _(raven for node)_
- [ ] other:

### Version:

```
5.14.0
```

## Description

Updating to 5.14.0 broke my web worker code when loading sentry.

TypeError: null is not an object (evaluating 'performance.timing.navigationStart')