Auto merge of #12839 - notriddle:11818_sequential_layout_bug, r=emilio

Fix a cached style cascade bug that only manifested in sequential mode

When copying cached styles, keep the `writing_mode` up to date.

---

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #11818 (github issue number if applicable).
- [X] There are tests for these changes

EDIT: The test is now working. I ran it with the first commit (the actual fix) reverted and it failed.

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components/constellation/pipeline.rs

@@ -162,7 +162,7 @@ impl Pipeline {
                     pipeline_port: pipeline_port,
                     layout_to_constellation_chan: state.layout_to_constellation_chan.clone(),
                     content_process_shutdown_chan: layout_content_process_shutdown_chan.clone(),
-                    layout_threads: opts::get().layout_threads,
+                    layout_threads: PREFS.get("layout.threads").as_u64().expect("count") as usize,
                 };
 
                 if let Err(e) = script_chan.send(ConstellationControlMsg::AttachLayout(new_layout_info)) {
@@ -474,7 +474,8 @@ impl UnprivilegedPipelineContent {
                     self.mem_profiler_chan,
                     self.layout_content_process_shutdown_chan,
                     self.webrender_api_sender,
-                    opts::get().layout_threads);
+                    self.prefs.get("layout.threads").expect("exists").value()
+                        .as_u64().expect("count") as usize);
 
         if wait_for_completion {
             let _ = self.script_content_process_shutdown_port.recv();