LibWeb: Ensure m_is_finished is false after cancelling Animation

Author: https://github.com/mattco98
Commit: fc6a6d29ec
Pull-request: https://github.com/SerenityOS/serenity/pull/23523

Userland/Libraries/LibWeb/Animations/Animation.cpp

@@ -430,6 +430,7 @@ void Animation::cancel(ShouldInvalidate should_invalidate)
 
         // 4. Let current finished promise be a new promise in the relevant Realm of animation.
         m_current_finished_promise = WebIDL::create_promise(realm);
+        m_is_finished = false;
 
         // 5. Create an AnimationPlaybackEvent, cancelEvent.
         // 6. Set cancelEvent’s type attribute to cancel.