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/shared/script/lib.rs

@@ -15,6 +15,7 @@ use std::sync::Arc;
 
 use background_hang_monitor_api::BackgroundHangMonitorRegister;
 use base::cross_process_instant::CrossProcessInstant;
+use base::generic_channel::{GenericReceiver, GenericSender};
 use base::id::{BrowsingContextId, HistoryStateId, PipelineId, PipelineNamespaceId, WebViewId};
 #[cfg(feature = "bluetooth")]
 use bluetooth_traits::BluetoothRequest;
@@ -307,9 +308,9 @@ pub struct InitialScriptState {
     /// Loading into a Secure Context
     pub inherited_secure_context: Option<bool>,
     /// A channel with which messages can be sent to us (the script thread).
-    pub constellation_sender: IpcSender<ScriptThreadMessage>,
+    pub constellation_sender: GenericSender<ScriptThreadMessage>,
     /// A port on which messages sent by the constellation to script can be received.
-    pub constellation_receiver: IpcReceiver<ScriptThreadMessage>,
+    pub constellation_receiver: GenericReceiver<ScriptThreadMessage>,
     /// A channel on which messages can be sent to the constellation from script.
     pub pipeline_to_constellation_sender: ScriptToConstellationChan,
     /// A handle to register script-(and associated layout-)threads for hang monitoring.