LibWeb: Unregister AnimationTimeline from document in finalize()

...instead of destructor because by the time it is executed, document
could already be destroyed.

Userland/Libraries/LibWeb/Animations/AnimationTimeline.cpp

@@ -48,7 +48,7 @@ AnimationTimeline::AnimationTimeline(JS::Realm& realm)
 {
 }
 
-AnimationTimeline::~AnimationTimeline()
+void AnimationTimeline::finalize()
 {
     if (m_associated_document)
         m_associated_document->disassociate_with_timeline(*this);

Userland/Libraries/LibWeb/Animations/AnimationTimeline.h

@@ -35,10 +35,10 @@ public:
 
 protected:
     AnimationTimeline(JS::Realm&);
-    virtual ~AnimationTimeline() override;
 
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
+    virtual void finalize() override;
 
     // https://www.w3.org/TR/web-animations-1/#dom-animationtimeline-currenttime
     Optional<double> m_current_time {};