servo: Merge #12560 - Hoist retrieval of layout_threads from opts into Constellation (from djc:layout-threads-hoist); r=emilio

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This makes the lower-level crates less dependent on `util::opts`.

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- [ ] These changes fix #__ (github issue number if applicable).

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Source-Repo: https://github.com/servo/servo
Source-Revision: 2d0178863de61f3cbe3ab24e193dff783dcfc9a3

servo/components/script_traits/lib.rs

@@ -146,6 +146,8 @@ pub struct NewLayoutInfo {
     pub layout_to_constellation_chan: IpcSender<LayoutMsg>,
     /// A shutdown channel so that layout can tell the content process to shut down when it's done.
     pub content_process_shutdown_chan: IpcSender<()>,
+    /// Number of threads to use for layout.
+    pub layout_threads: usize,
 }
 
 /// Messages sent from the constellation or layout to the script thread.