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/layout/layout_impl.rs

@@ -13,6 +13,7 @@ use std::sync::{Arc, LazyLock};
 
 use app_units::Au;
 use base::Epoch;
+use base::generic_channel::GenericSender;
 use base::id::{PipelineId, WebViewId};
 use bitflags::bitflags;
 use compositing_traits::CrossProcessCompositorApi;
@@ -25,7 +26,6 @@ use fnv::FnvHashMap;
 use fonts::{FontContext, FontContextWebFontMethods};
 use fonts_traits::StylesheetWebFontLoadFinishedCallback;
 use fxhash::FxHashMap;
-use ipc_channel::ipc::IpcSender;
 use layout_api::wrapper_traits::LayoutNode;
 use layout_api::{
     IFrameSizes, Layout, LayoutConfig, LayoutDamage, LayoutFactory, OffsetParentResponse,
@@ -135,7 +135,7 @@ pub struct LayoutThread {
     is_iframe: bool,
 
     /// The channel on which messages can be sent to the script thread.
-    script_chan: IpcSender<ScriptThreadMessage>,
+    script_chan: GenericSender<ScriptThreadMessage>,
 
     /// The channel on which messages can be sent to the time profiler.
     time_profiler_chan: profile_time::ProfilerChan,