Add stubs for session start/end

components/script/dom/xrsession.rs

@@ -288,6 +288,14 @@ impl XRSessionMethods for XRSession {
             .map(|x| DomRoot::from_ref(&**x))
             .collect()
     }
+
+    /// https://immersive-web.github.io/webxr/#dom-xrsession-end
+    fn End(&self) -> Rc<Promise> {
+        // XXXManishearth implement device disconnection and session ending
+        let p = Promise::new(&self.global());
+        p.resolve_native(&());
+        p
+    }
 }
 
 #[derive(Clone, Copy, Debug)]