LibWeb: Add a default DocumentTimeline object to DOM::Document

Author: https://github.com/mattco98
Commit: 2fbb9649e3
Pull-request: https://github.com/SerenityOS/serenity/pull/21801
Reviewed-by: https://github.com/awesomekling ✅

Userland/Libraries/LibWeb/DOM/Document.h

@@ -534,6 +534,8 @@ public:
 
     HashMap<AK::URL, HTML::SharedImageRequest*>& shared_image_requests();
 
+    JS::NonnullGCPtr<Animations::DocumentTimeline> timeline();
+
 protected:
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
@@ -740,6 +742,9 @@ private:
     JS::GCPtr<HTML::SessionHistoryEntry> m_latest_entry;
 
     HashMap<AK::URL, HTML::SharedImageRequest*> m_shared_image_requests;
+
+    // https://www.w3.org/TR/web-animations-1/#document-default-document-timeline
+    JS::GCPtr<Animations::DocumentTimeline> m_default_timeline;
 };
 
 template<>