servo: Merge #16014 - Per-process lock for CSSOM objects (from servo:style-ref); r=emilio

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Before this PR, every object reflected in CSSOM is in `Arc<RwLock<_>>` to enable safe (synchronized) mutable aliasing. Acquiring all these locks has significant cost during selector matching:

* https://bugzilla.mozilla.org/show_bug.cgi?id=1311469
* https://bugzilla.mozilla.org/show_bug.cgi?id=1335941
* https://bugzilla.mozilla.org/show_bug.cgi?id=1339703

This PR introduce a mechanism to protect many objects with the same `RwLock` that only needs to be acquired once.

In Stylo, there is one such lock per process (in a `lazy_static`), used for everything.

I non-Stylo Servo, I originally intended to have one such lock per document (for author-origin stylesheets, and one per process for user-agent and user sytlesheets since they’re shared across documents, and never mutated anyway). However I failed to have the same document-specific (or pipeline-specific) `Arc` reachable from both `Document` nodes and `LayoutThread`. Recursively following callers lead me to include this `Arc` in `UnprivilegedPipelineContent`, but that needs to be serializable. So there is a second process-wide lock.

This was previously #15998, closed accidentally.

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Source-Repo: https://github.com/servo/servo
Source-Revision: bb54f0a429de0e8b8861f8071b6cf82f73622664

servo/components/script/dom/cssrule.rs

@@ -20,6 +20,7 @@ use dom::cssviewportrule::CSSViewportRule;
 use dom::window::Window;
 use dom_struct::dom_struct;
 use std::cell::Cell;
+use style::shared_lock::SharedRwLock;
 use style::stylesheets::CssRule as StyleCssRule;
 
 
@@ -103,6 +104,10 @@ impl CSSRule {
     pub fn parent_stylesheet(&self) -> &CSSStyleSheet {
         &self.parent_stylesheet
     }
+
+    pub fn shared_lock(&self) -> &SharedRwLock {
+        &self.parent_stylesheet.style_stylesheet().shared_lock
+    }
 }
 
 impl CSSRuleMethods for CSSRule {