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/dom/window.rs

@@ -17,6 +17,7 @@ use std::time::{Duration, Instant};
 use app_units::Au;
 use backtrace::Backtrace;
 use base::cross_process_instant::CrossProcessInstant;
+use base::generic_channel::GenericSender;
 use base::id::{BrowsingContextId, PipelineId, WebViewId};
 use base64::Engine;
 #[cfg(feature = "bluetooth")]
@@ -3043,7 +3044,7 @@ impl Window {
         time_profiler_chan: TimeProfilerChan,
         devtools_chan: Option<IpcSender<ScriptToDevtoolsControlMsg>>,
         constellation_chan: ScriptToConstellationChan,
-        control_chan: IpcSender<ScriptThreadMessage>,
+        control_chan: GenericSender<ScriptThreadMessage>,
         pipeline_id: PipelineId,
         parent_info: Option<PipelineId>,
         viewport_details: ViewportDetails,
@@ -3292,7 +3293,7 @@ impl Window {
 #[derive(MallocSizeOf)]
 pub(crate) struct CSSErrorReporter {
     pub(crate) pipelineid: PipelineId,
-    pub(crate) script_chan: IpcSender<ScriptThreadMessage>,
+    pub(crate) script_chan: GenericSender<ScriptThreadMessage>,
 }
 unsafe_no_jsmanaged_fields!(CSSErrorReporter);
 

components/script/messaging.rs

@@ -8,6 +8,7 @@ use std::cell::RefCell;
 use std::option::Option;
 use std::result::Result;
 
+use base::generic_channel::GenericSender;
 use base::id::PipelineId;
 #[cfg(feature = "bluetooth")]
 use bluetooth_traits::BluetoothRequest;
@@ -331,7 +332,7 @@ pub(crate) struct ScriptThreadSenders {
 
     /// A [`Sender`] that sends messages to the `Constellation`.
     #[no_trace]
-    pub(crate) constellation_sender: IpcSender<ScriptThreadMessage>,
+    pub(crate) constellation_sender: GenericSender<ScriptThreadMessage>,
 
     /// A [`Sender`] that sends messages to the `Constellation` associated with
     /// particular pipelines.

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;