Rollup merge of #17014 - hiikezoe:animation-rules-in-norma-restyle, r=emilio,birtles

Animation rules in norma restyle

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This is a PR for https://bugzilla.mozilla.org/show_bug.cgi?id=1366631 and https://bugzilla.mozilla.org/show_bug.cgi?id=1367225

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because it's for stylo

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components/style/properties/declaration_block.rs

@@ -11,6 +11,7 @@ use cssparser::{DeclarationListParser, parse_important};
 use cssparser::{Parser, AtRuleParser, DeclarationParser, Delimiter};
 use error_reporting::ParseErrorReporter;
 use parser::{PARSING_MODE_DEFAULT, ParsingMode, ParserContext, log_css_error};
+use shared_lock::Locked;
 use std::fmt;
 use std::slice::Iter;
 use style_traits::ToCss;
@@ -18,6 +19,20 @@ use stylesheets::{CssRuleType, Origin, UrlExtraData};
 use super::*;
 #[cfg(feature = "gecko")] use properties::animated_properties::AnimationValueMap;
 
+/// The animation rules.
+///
+/// The first one is for Animation cascade level, and the second one is for
+/// Transition cascade level.
+pub struct AnimationRules<'a>(pub Option<&'a Arc<Locked<PropertyDeclarationBlock>>>,
+                              pub Option<&'a Arc<Locked<PropertyDeclarationBlock>>>);
+
+impl<'a> AnimationRules<'a> {
+    /// Returns whether these animation rules represents an actual rule or not.
+    pub fn is_empty(&self) -> bool {
+        self.0.is_none() && self.1.is_none()
+    }
+}
+
 /// A declaration [importance][importance].
 ///
 /// [importance]: https://drafts.csswg.org/css-cascade/#importance