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/layout_traits/lib.rs

@@ -43,7 +43,6 @@ pub trait LayoutThreadFactory {
         content_process_shutdown_chan: Option<IpcSender<()>>,
         webrender_api_sender: webrender_api::RenderApiSender,
         webrender_document: webrender_api::DocumentId,
-        layout_threads: usize,
         paint_time_metrics: PaintTimeMetrics,
     );
 }