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/constellation/event_loop.rs

@@ -11,15 +11,15 @@ use std::marker::PhantomData;
 use std::rc::Rc;
 use std::sync::atomic::{AtomicUsize, Ordering};
 
+use base::generic_channel::GenericSender;
 use ipc_channel::Error;
-use ipc_channel::ipc::IpcSender;
 use script_traits::ScriptThreadMessage;
 
 static CURRENT_EVENT_LOOP_ID: AtomicUsize = AtomicUsize::new(0);
 
 /// <https://html.spec.whatwg.org/multipage/#event-loop>
 pub struct EventLoop {
-    script_chan: IpcSender<ScriptThreadMessage>,
+    script_chan: GenericSender<ScriptThreadMessage>,
     dont_send_or_sync: PhantomData<Rc<()>>,
     id: usize,
 }
@@ -46,7 +46,7 @@ impl Drop for EventLoop {
 
 impl EventLoop {
     /// Create a new event loop from the channel to its script thread.
-    pub fn new(script_chan: IpcSender<ScriptThreadMessage>) -> Rc<EventLoop> {
+    pub fn new(script_chan: GenericSender<ScriptThreadMessage>) -> Rc<EventLoop> {
         let id = CURRENT_EVENT_LOOP_ID.fetch_add(1, Ordering::Relaxed);
         Rc::new(EventLoop {
             script_chan,
@@ -57,6 +57,8 @@ impl EventLoop {
 
     /// Send a message to the event loop.
     pub fn send(&self, msg: ScriptThreadMessage) -> Result<(), Error> {
-        self.script_chan.send(msg)
+        self.script_chan
+            .send(msg)
+            .map_err(|_err| Box::new(ipc_channel::ErrorKind::Custom("SendError".into())))
     }
 }