Specify behavior for browsers that do not want to reveal screen plurality via Screen.isExtended

[michaelwasserman]

This was discussed in mozilla/standards-positions#542.

The Screen.isExtended bit is an important boolean signal for the site to know whether it is worth asking the user for additional screen details at all. That said, we want to allow different browsers to adopt different permission models. So, the spec should probably state what it looks like if the browser doesn't want to reveal the plurality of screens via Screen.isExtended. Perhaps it could be left undefined or always set to true, requiring or encouraging explicit async requests to equivalent information via Window.getScreenDetails().