Reuse StylesheetContent for inline style sheets with identical content (#38540)

For duplicate style sheets with identical content, `StylesheetContents`
can be reused to avoid redundant parsing of the inline style sheets.
Since duplicate stylesheets is a common case with web components, this
change will significantly improve performance. Additionally, the cache
hit rate of stylo's `CascadeDataCache` can now be significantly
improved.

When shared `StylesheetContents` is modified, copy-on-write will occur
to avoid affecting other sharers. And then updates the references to
`CssRule` or `PropertyDeclarationBlock` stored in the CSSOMs to ensure
that modifications are made only on the new copy.

Signed-off-by: sharpshooter_pt <ibluegalaxy_taoj@163.com>

components/script/dom/cssstyledeclaration.rs

@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use std::cell::RefCell;
 use std::cmp::Ordering;
 use std::sync::LazyLock;
 
@@ -53,7 +54,7 @@ pub(crate) enum CSSStyleOwner {
         Dom<CSSRule>,
         #[ignore_malloc_size_of = "Arc"]
         #[no_trace]
-        Arc<Locked<PropertyDeclarationBlock>>,
+        RefCell<Arc<Locked<PropertyDeclarationBlock>>>,
     ),
 }
 
@@ -119,9 +120,10 @@ impl CSSStyleOwner {
                 result
             },
             CSSStyleOwner::CSSRule(ref rule, ref pdb) => {
+                rule.parent_stylesheet().will_modify();
                 let result = {
                     let mut guard = rule.shared_lock().write();
-                    f(&mut *pdb.write_with(&mut guard), &mut changed)
+                    f(&mut *pdb.borrow().write_with(&mut guard), &mut changed)
                 };
                 if changed {
                     rule.parent_stylesheet().notify_invalidations();
@@ -152,7 +154,7 @@ impl CSSStyleOwner {
             },
             CSSStyleOwner::CSSRule(ref rule, ref pdb) => {
                 let guard = rule.shared_lock().read();
-                f(pdb.read_with(&guard))
+                f(pdb.borrow().read_with(&guard))
             },
         }
     }
@@ -265,6 +267,17 @@ impl CSSStyleDeclaration {
         )
     }
 
+    pub(crate) fn update_property_declaration_block(
+        &self,
+        pdb: &Arc<Locked<PropertyDeclarationBlock>>,
+    ) {
+        if let CSSStyleOwner::CSSRule(_, pdb_cell) = &self.owner {
+            *pdb_cell.borrow_mut() = pdb.clone();
+        } else {
+            panic!("update_rule called on CSSStyleDeclaration with a Element owner");
+        }
+    }
+
     fn get_computed_style(&self, property: PropertyId) -> DOMString {
         match self.owner {
             CSSStyleOwner::CSSRule(..) => {