Add support for canceling CSS transitions

This change adds support for canceling CSS transitions when a property
is no longer transitionable or when an element becomes styled with
display:none. Support for canceling and replacing CSS transitions when
the end value changes is still pending. This change also takes advantage
of updating the constellation message to fix a bug where transition
events could be sent for closed pipelines.

Fixes #15079.

components/layout_thread_2020/lib.rs

@@ -609,6 +609,7 @@ impl LayoutThread {
                 visited_styles_enabled: false,
                 running_animations: Default::default(),
                 expired_animations: Default::default(),
+                cancelled_animations: Default::default(),
                 registered_speculative_painters: &self.registered_painters,
                 local_context_creation_data: Mutex::new(thread_local_style_context_creation_data),
                 timer: self.timer.clone(),