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/base/Cargo.toml

@@ -20,6 +20,7 @@ malloc_size_of = { workspace = true }
 malloc_size_of_derive = { workspace = true }
 parking_lot = { workspace = true }
 serde = { workspace = true }
+servo_config = { path = "../../config" }
 time = { workspace = true }
 webrender_api = { workspace = true }
 

components/shared/base/generic_channel.rs

@@ -7,6 +7,7 @@
 use std::fmt;
 
 use ipc_channel::router::ROUTER;
+use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
 static GENERIC_CHANNEL_USAGE_ERROR_PANIC_MSG: &str = "May not send a crossbeam channel over an IPC channel. \
@@ -65,6 +66,12 @@ impl<T: Serialize> GenericSender<T> {
     }
 }
 
+impl<T: Serialize> MallocSizeOf for GenericSender<T> {
+    fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
+        0
+    }
+}
+
 #[derive(Debug)]
 pub struct SendError;
 pub type SendResult = Result<(), SendError>;
@@ -145,11 +152,11 @@ where
 /// Creates a Servo channel that can select different channel implementations based on multiprocess
 /// mode or not. If the scenario doesn't require message to pass process boundary, a simple
 /// crossbeam channel is preferred.
-pub fn channel<T>(multiprocess: bool) -> Option<(GenericSender<T>, GenericReceiver<T>)>
+pub fn channel<T>() -> Option<(GenericSender<T>, GenericReceiver<T>)>
 where
     T: for<'de> Deserialize<'de> + Serialize,
 {
-    if multiprocess {
+    if servo_config::opts::get().multiprocess {
         ipc_channel::ipc::channel()
             .map(|(tx, rx)| (GenericSender::Ipc(tx), GenericReceiver::Ipc(rx)))
             .ok()