Auto merge of #26594 - mrobinson:animationiteration, r=jdm

Implement animationiteration event

This event is triggered when an animation iterates. This change also
moves iteration out of style calculation to an "update animations" which
is the next part of having animation event handling match the HTML spec.

This change causes a few more tests to pass. Some of the other tests which
exercise this functionality require `animationstart` events.

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- [ ] These changes fix #___ (GitHub issue number if applicable)
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components/script/dom/webidls/EventHandler.webidl

@@ -93,6 +93,7 @@ interface mixin GlobalEventHandlers {
 // https://drafts.csswg.org/css-animations/#interface-globaleventhandlers-idl
 partial interface mixin GlobalEventHandlers {
            attribute EventHandler onanimationend;
+           attribute EventHandler onanimationiteration;
 };
 
 // https://drafts.csswg.org/css-transitions/#interface-globaleventhandlers-idl