Auto merge of #22389 - jdm:anim-complete, r=emilio,ferjm

Cancel animations that affect nodes that do not participate in layout.

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #22378
- [x] There are tests for these changes

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components/script/dom/webidls/Window.webidl

@@ -173,3 +173,8 @@ partial interface Window {
    readonly attribute TestRunner testRunner;
    //readonly attribute EventSender eventSender;
 };
+
+partial interface Window {
+   [Pref="css.animations.testing.enabled"]
+   readonly attribute unsigned long runningAnimationCount;
+};