net: Use callbacks instead of IPC channels in the ImageCache API (#40453)

Previously, the `ImageCache` would inform consumers of changes to image
availability by messaging an IPC channel. This isn't great, because it
requires serialization and deserialization. Nowadays, `ImageCache`s are
always owned by the same `Global` that uses them. This change replaces
the IPC channel with callback that implements `Send`.

For the case of normal HTML document images, the results are still sent
over the pre-existing Crossbeam channel that was connected via an IPC
route. A followup change might eliminate that channel entirely.

Testing: This should not change observable behavior so is covered by
existing tests.
Fixes: #24338.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Mukilan Thiyagarajan <mukilan@igalia.com>

components/script/script_thread.rs

@@ -896,13 +896,6 @@ impl ScriptThread {
         );
 
         let (image_cache_sender, image_cache_receiver) = unbounded();
-        let (ipc_image_cache_sender, ipc_image_cache_receiver) = ipc::channel().unwrap();
-        ROUTER.add_typed_route(
-            ipc_image_cache_receiver,
-            Box::new(move |message| {
-                let _ = image_cache_sender.send(message.unwrap());
-            }),
-        );
 
         let receivers = ScriptThreadReceivers {
             constellation_receiver,
@@ -921,7 +914,7 @@ impl ScriptThread {
             constellation_sender: state.constellation_sender,
             pipeline_to_constellation_sender: state.pipeline_to_constellation_sender.sender.clone(),
             pipeline_to_embedder_sender: state.pipeline_to_embedder_sender.clone(),
-            image_cache_sender: ipc_image_cache_sender,
+            image_cache_sender,
             time_profiler_sender: state.time_profiler_sender,
             memory_profiler_sender: state.memory_profiler_sender,
             devtools_server_sender,