[web-animations] Implement KeyframeEffect.iterationComposite

https://bugs.webkit.org/show_bug.cgi?id=246851
rdar://101423139

Reviewed by Antti Koivisto.

Now that all blocking bugs have been resolved and all related WPT tests pass, we can enable
KeyframeEffect.iterationComposite by default.

* Source/WTF/Scripts/Preferences/WebPreferencesExperimental.yaml:

Canonical link: https://commits.webkit.org/257059@main

Source/WTF/Scripts/Preferences/WebPreferencesExperimental.yaml

@@ -1890,11 +1890,11 @@ WebAnimationsIterationCompositeEnabled:
   humanReadableDescription: "Support for the KeyframeEffect.iterationComposite property"
   defaultValue:
     WebKitLegacy:
-      default: false
+      default: true
     WebKit:
-      default: false
+      default: true
     WebCore:
-      default: false
+      default: true
 
 WebAnimationsMutableTimelinesEnabled:
   type: bool