script: Move Window-only gamepad functionality out of GlobalScope (#36805)

The only code that calls these methods is in the script thread, and the
code is simpler when we can assume a Window global. Pulling this thread
led to cleaning up a lot of constructors for Window-only WebXR code,
too.

Testing: Existing WPT coverage.

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script/dom/webxr/xrsystem.rs

@@ -297,7 +297,13 @@ impl XRSystem {
                 return;
             },
         };
-        let session = XRSession::new(&self.global(), session, mode, frame_receiver, CanGc::note());
+        let session = XRSession::new(
+            self.global().as_window(),
+            session,
+            mode,
+            frame_receiver,
+            CanGc::note(),
+        );
         if mode == XRSessionMode::Inline {
             self.active_inline_sessions
                 .borrow_mut()