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/layout/lib.rs

@@ -20,6 +20,7 @@ use std::thread::JoinHandle;
 use app_units::Au;
 use atomic_refcell::AtomicRefCell;
 use base::Epoch;
+use base::generic_channel::GenericSender;
 use base::id::{BrowsingContextId, PipelineId, WebViewId};
 use bitflags::bitflags;
 use compositing_traits::CrossProcessCompositorApi;
@@ -30,7 +31,6 @@ use euclid::default::{Point2D as UntypedPoint2D, Rect};
 use fnv::FnvHashMap;
 use fonts::{FontContext, SystemFontServiceProxy};
 use fxhash::FxHashMap;
-use ipc_channel::ipc::IpcSender;
 pub use layout_damage::LayoutDamage;
 use libc::c_void;
 use malloc_size_of::{MallocSizeOf as MallocSizeOfTrait, MallocSizeOfOps, malloc_size_of_is_0};
@@ -197,7 +197,7 @@ pub struct LayoutConfig {
     pub webview_id: WebViewId,
     pub url: ServoUrl,
     pub is_iframe: bool,
-    pub script_chan: IpcSender<ScriptThreadMessage>,
+    pub script_chan: GenericSender<ScriptThreadMessage>,
     pub image_cache: Arc<dyn ImageCache>,
     pub font_context: Arc<FontContext>,
     pub time_profiler_chan: time::ProfilerChan,