Remove usage of unstable box syntax, except in the script crate

… because there’s a lot of it,
and script still uses any other unstable features anyway.

components/devtools/actors/framerate.rs

@@ -52,7 +52,7 @@ impl FramerateActor {
         };
 
         actor.start_recording();
-        registry.register_later(box actor);
+        registry.register_later(Box::new(actor));
         actor_name
     }