script: Do not set up an IPC route for every image load (#35041)

Instead of setting up a route for every image load in the DOM / Layout,
route all incoming image cache responses through the `ScriptThread`.
This avoids creating a set of file descriptor for every image that is
loaded.

This change requires having the `ImageCache` track the `PipelineId` of
the original the listener so that the `ScriptThread` can route it
properly to the correct `Window`.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/globalscope.rs

@@ -233,7 +233,6 @@ pub(crate) struct GlobalScope {
     inline_module_map: DomRefCell<HashMap<ScriptId, Rc<ModuleTree>>>,
 
     /// For providing instructions to an optional devtools server.
-    #[ignore_malloc_size_of = "channels are hard"]
     #[no_trace]
     devtools_chan: Option<IpcSender<ScriptToDevtoolsControlMsg>>,