Creation of web workers from cross origin sources is not allowed unfortunately.

We need to instead

• fetch the source via a CORS request
• create a blob
• launch the Worker with that.

See e.g. this example https://gist.github.com/roman01la/e7a7b879698574c45ca0

This requires the source itself to be CORS accessible, which, coincidentally I need to do anyway because of #7353. See cr/147554802 for that change.