Start having animations conform to the HTML spec

This is a small step toward fixing #19242. The main idea is that the
clock for animations should advance as the event loop ticks. We
accomplish this by moving the clock from layout and naming it the
"animation timeline" which is the spec language. This should fix
flakiness with animations and transitions tests where a reflow could
move animations forward while script was running.

This change also starts to break out transition and animation events
into their own data structure, because it's quite likely that the next
step in fixing #19242 is to no longer send these events through a
channel.

components/script/dom/document.rs

@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use crate::animation_timeline::AnimationTimeline;
 use crate::document_loader::{DocumentLoader, LoadType};
 use crate::dom::attr::Attr;
 use crate::dom::beforeunloadevent::BeforeUnloadEvent;
@@ -380,6 +381,9 @@ pub struct Document {
     csp_list: DomRefCell<Option<CspList>>,
     /// https://w3c.github.io/slection-api/#dfn-selection
     selection: MutNullableDom<Selection>,
+    /// A timeline for animations which is used for synchronizing animations.
+    /// https://drafts.csswg.org/web-animations/#timeline
+    animation_timeline: DomRefCell<AnimationTimeline>,
 }
 
 #[derive(JSTraceable, MallocSizeOf)]
@@ -2904,6 +2908,11 @@ impl Document {
             dirty_webgl_contexts: DomRefCell::new(HashMap::new()),
             csp_list: DomRefCell::new(None),
             selection: MutNullableDom::new(None),
+            animation_timeline: if pref!(layout.animations.test.enabled) {
+                DomRefCell::new(AnimationTimeline::new_for_testing())
+            } else {
+                DomRefCell::new(AnimationTimeline::new())
+            },
         }
     }
 
@@ -3605,6 +3614,18 @@ impl Document {
             })
             .collect()
     }
+
+    pub fn advance_animation_timeline_for_testing(&self, delta: f64) {
+        self.animation_timeline.borrow_mut().advance_specific(delta);
+    }
+
+    pub fn update_animation_timeline(&self) {
+        self.animation_timeline.borrow_mut().update();
+    }
+
+    pub fn current_animation_timeline_value(&self) -> f64 {
+        self.animation_timeline.borrow().current_value()
+    }
 }
 
 impl Element {