Auto merge of #26244 - mrobinson:animation-cancel, r=jdm

Add support for canceling CSS transitions

This change adds support for canceling CSS transitions when a property is
no longer transitionable. 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.

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- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15079.

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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(),