style: Add telemetry for -moz-appearance usage.

Differential Revision: https://phabricator.services.mozilla.com/D77637

components/style/properties/declaration_block.rs

@@ -348,6 +348,21 @@ impl PropertyDeclarationBlock {
             .find(|(declaration, _)| declaration.id() == property)
     }
 
+    /// Get a declaration for a given property with the specified importance.
+    #[inline]
+    pub fn get_at_importance(
+        &self,
+        property: PropertyDeclarationId,
+        importance: Importance,
+    ) -> Option<&PropertyDeclaration> {
+        let (declaration, i) = self.get(property)?;
+        if i == importance {
+            Some(declaration)
+        } else {
+            None
+        }
+    }
+
     /// Tries to serialize a given shorthand from the declarations in this
     /// block.
     pub fn shorthand_to_css(