Auto merge of #26921 - mrobinson:animation-set-key, r=jdm

Add DocumentAnimationSet and AnimationSetKey

This will be used in order to hold animations for pseudo elements in the
DocumentAnimationSet. Also no longer store the OpaqueNode in the
animation and transition data structures. This is already part of the
DocumentAnimationSet key.

---
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components/layout_2020/flow/root.rs

@@ -35,6 +35,7 @@ use gfx_traits::print_tree::PrintTree;
 use script_layout_interface::wrapper_traits::LayoutNode;
 use script_layout_interface::{LayoutElementType, LayoutNodeType};
 use servo_arc::Arc;
+use style::animation::AnimationSetKey;
 use style::dom::OpaqueNode;
 use style::properties::ComputedValues;
 use style::values::computed::Length;
@@ -446,10 +447,10 @@ impl FragmentTree {
         })
     }
 
-    pub fn remove_nodes_in_fragment_tree_from_set(&self, set: &mut FxHashSet<OpaqueNode>) {
+    pub fn remove_nodes_in_fragment_tree_from_set(&self, set: &mut FxHashSet<AnimationSetKey>) {
         self.find(|fragment, _| {
             if let Some(tag) = fragment.tag().as_ref() {
-                set.remove(&tag.node());
+                set.remove(&AnimationSetKey(tag.node()));
             }
             None::<()>
         });