Don't throw on screen getters.

The screen lifetime seems like it was a lot more complicated in the past (looking at the comments in Window.webidl).

But nowadays it seems it can be more similar to the other objects like VisualViewport etc.

Differential Revision: https://phabricator.services.mozilla.com/D193212

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1858977 gecko-commit: bf2dfd2da6343755ec14d27b12d3e33c118066da gecko-reviewers: smaug, webidl