style: Make Servo use a single thread-pool for layout-related tasks per-process.

Instead of per-document. This also allows to reuse this thread-pool if needed
for other stuff, like parallel CSS parsing (#22478), and to share more code with
Gecko, which is always nice.

components/script_layout_interface/message.rs

@@ -220,6 +220,5 @@ pub struct NewLayoutThreadInfo {
     pub script_chan: IpcSender<ConstellationControlMsg>,
     pub image_cache: Arc<dyn ImageCache>,
     pub content_process_shutdown_chan: Option<IpcSender<()>>,
-    pub layout_threads: usize,
     pub paint_time_metrics: PaintTimeMetrics,
 }