Auto merge of #15535 - froydnj:global-style-thread-pool, r=bholley

geckolib: use a global thread pool for styling

By having a single thread pool, rather than one per document, we use less memory.  This addresses https://bugzilla.mozilla.org/show_bug.cgi?id=1324250.

This may be obvious to an experienced Rust programmer, but I went with raw pointers because trying to use `Option` global variables resulted in complaints about turning on feature flags in nightly Rust.  Since this is for stylo, nightly features are not appropriate here.

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

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tests/unit/stylo/Cargo.toml

@@ -22,6 +22,7 @@ libc = "0.2"
 log = {version = "0.3.5", features = ["release_max_level_info"]}
 num_cpus = "1.1.0"
 parking_lot = "0.3"
+rayon = "0.6"
 selectors = {path = "../../../components/selectors"}
 servo_url = {path = "../../../components/url"}
 style_traits = {path = "../../../components/style_traits"}

tests/unit/stylo/lib.rs

@@ -8,8 +8,11 @@ extern crate cssparser;
 extern crate env_logger;
 extern crate euclid;
 extern crate geckoservo;
+#[macro_use] extern crate lazy_static;
 #[macro_use] extern crate log;
+extern crate num_cpus;
 extern crate parking_lot;
+extern crate rayon;
 extern crate selectors;
 extern crate servo_url;
 extern crate style;