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/pipeline.rs

@@ -12,6 +12,7 @@ use background_hang_monitor_api::{
     BackgroundHangMonitorControlMsg, BackgroundHangMonitorRegister, HangMonitorAlert,
 };
 use base::Epoch;
+use base::generic_channel::{GenericReceiver, GenericSender};
 use base::id::{
     BrowsingContextId, HistoryStateId, PipelineId, PipelineNamespace, PipelineNamespaceId,
     PipelineNamespaceRequest, WebViewId,
@@ -239,7 +240,8 @@ impl Pipeline {
                 (script_chan, (None, None, None))
             },
             None => {
-                let (script_chan, script_port) = ipc::channel().expect("Pipeline script chan");
+                let (script_chan, script_port) =
+                    base::generic_channel::channel().expect("Pipeline script chan");
 
                 // Route messages coming from content to devtools as appropriate.
                 let script_to_devtools_ipc_sender =
@@ -482,9 +484,9 @@ pub struct UnprivilegedPipelineContent {
     mem_profiler_chan: profile_mem::ProfilerChan,
     viewport_details: ViewportDetails,
     theme: Theme,
-    script_chan: IpcSender<ScriptThreadMessage>,
+    script_chan: GenericSender<ScriptThreadMessage>,
     load_data: LoadData,
-    script_port: IpcReceiver<ScriptThreadMessage>,
+    script_port: GenericReceiver<ScriptThreadMessage>,
     opts: Opts,
     prefs: Box<Preferences>,
     pipeline_namespace_id: PipelineNamespaceId,