Do not create `self.Worker` in a window.

`self.Worker` needs to be created in a web worker; in particular
`self.Worker.postMessage` is needed.  It is also needed in windows that
support web workers but lack `MessageChannel`.

However, for browsers that do not even support web workers, we do not want to 
create a `Worker` object as this may give false positives to feature
detection.

@Cyril-sf this look okay?