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/style/lib.rs

@@ -133,6 +133,7 @@ pub mod font_metrics;
 #[cfg(feature = "gecko")]
 #[allow(unsafe_code)]
 pub mod gecko_bindings;
+pub mod global_style_data;
 pub mod hash;
 pub mod invalidation;
 #[allow(missing_docs)] // TODO.