Auto merge of #15943 - hiikezoe:animation-on-pseudo, r=heycam

Use computed styles of the pseudo and its parent when calling Gecko_U…

…pdateAnimations to update CSS Animations on pseudo-elements.

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This is a PR of https://bugzilla.mozilla.org/show_bug.cgi?id=1346408 .

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- [X] `./mach test-tidy` does not report any errors

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- [X] These changes do not require tests because it's for stylo

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components/style/gecko/wrapper.rs

@@ -506,24 +506,31 @@ impl<'le> TElement for GeckoElement<'le> {
     }
 
     fn update_animations(&self, pseudo: Option<&PseudoElement>) {
-        let atom_ptr = PseudoElement::ns_atom_or_null_from_opt(pseudo);
-
         // We have to update animations even if the element has no computed style
         // since it means the element is in a display:none subtree, we should destroy
         // all CSS animations in display:none subtree.
         let computed_data = self.borrow_data();
-        let computed_values = computed_data.as_ref().map(|d| d.styles().primary.values());
+        let computed_values =
+            computed_data.as_ref().map(|d|
+                pseudo.map_or_else(|| d.styles().primary.values(),
+                                   |p| d.styles().pseudos.get(p).unwrap().values())
+            );
         let computed_values_opt = computed_values.map(|v|
             *HasArcFFI::arc_as_borrowed(v)
         );
 
-        let parent_element = self.parent_element();
+        let parent_element = if pseudo.is_some() {
+            self.parent_element()
+        } else {
+            Some(*self)
+        };
         let parent_data = parent_element.as_ref().and_then(|e| e.borrow_data());
         let parent_values = parent_data.as_ref().map(|d| d.styles().primary.values());
         let parent_values_opt = parent_values.map(|v|
             *HasArcFFI::arc_as_borrowed(v)
         );
 
+        let atom_ptr = PseudoElement::ns_atom_or_null_from_opt(pseudo);
         unsafe {
             Gecko_UpdateAnimations(self.0, atom_ptr,
                                    computed_values_opt,