Use GenericChannel for script_chan (#38645)

Motivation: 
Using our GenericChannel abstraction allows us to optimize IPC in
single-process mode to just use cross-beam channel.
To keep the diff low, and get early feedback, this PR only tackles a
single channel, but the intention is to port all ipc channels to the
generic channel, which allows us to skip serializing and deserializing
messages in single process mode.

Based on: 
- https://github.com/servo/servo/pull/38638
- https://github.com/servo/servo/pull/38636

Testing: Covered by existing tests

---------

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

components/script/script_thread.rs

@@ -871,9 +871,7 @@ impl ScriptThread {
             JS_AddInterruptCallback(cx, Some(interrupt_callback));
         }
 
-        // Ask the router to proxy IPC messages from the control port to us.
-        let constellation_receiver =
-            ROUTER.route_ipc_receiver_to_new_crossbeam_receiver(state.constellation_receiver);
+        let constellation_receiver = state.constellation_receiver.into_inner();
 
         // Ask the router to proxy IPC messages from the devtools to us.
         let devtools_server_sender = state.devtools_server_sender;