Record whether an snapshot is recording a class attribute change or id change.

I'll use this information in order to get fewer dependencies out of the
dependency set.

Bug: 1368240
MozReview-Commit-ID: 5HlmKmSNO8p

components/style/servo/selector_parser.rs

@@ -545,6 +545,12 @@ pub struct ServoElementSnapshot {
     pub attrs: Option<Vec<(AttrIdentifier, AttrValue)>>,
     /// Whether this element is an HTML element in an HTML document.
     pub is_html_element_in_html_document: bool,
+    /// Whether the class attribute changed or not.
+    pub class_changed: bool,
+    /// Whether the id attribute changed or not.
+    pub id_changed: bool,
+    /// Whether other attributes other than id or class changed or not.
+    pub other_attributes_changed: bool,
 }
 
 impl ServoElementSnapshot {
@@ -554,9 +560,27 @@ impl ServoElementSnapshot {
             state: None,
             attrs: None,
             is_html_element_in_html_document: is_html_element_in_html_document,
+            class_changed: false,
+            id_changed: false,
+            other_attributes_changed: false,
         }
     }
 
+    /// Returns whether the id attribute changed or not.
+    pub fn id_changed(&self) -> bool {
+        self.id_changed
+    }
+
+    /// Returns whether the class attribute changed or not.
+    pub fn class_changed(&self) -> bool {
+        self.class_changed
+    }
+
+    /// Returns whether other attributes other than id or class changed or not.
+    pub fn other_attr_changed(&self) -> bool {
+        self.other_attributes_changed
+    }
+
     fn get_attr(&self, namespace: &Namespace, name: &LocalName) -> Option<&AttrValue> {
         self.attrs.as_ref().unwrap().iter()
             .find(|&&(ref ident, _)| ident.local_name == *name &&