LibWeb: Use Core::Timer instead for animation driver timer

Platform::Timer uses JS::SafeFunction that prevents document from ever
being deallocated because of strong reference.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -4023,7 +4023,7 @@ void Document::ensure_animation_timer()
 {
     constexpr static auto timer_delay_ms = 1000 / 60;
     if (!m_animation_driver_timer) {
-        m_animation_driver_timer = Platform::Timer::create_repeating(timer_delay_ms, [this] {
+        m_animation_driver_timer = MUST(Core::Timer::create_repeating(timer_delay_ms, [this] {
             bool has_animations = false;
             for (auto& timeline : m_associated_animation_timelines) {
                 if (!timeline->associated_animations().is_empty()) {
@@ -4042,7 +4042,7 @@ void Document::ensure_animation_timer()
                 for (auto& animation : timeline->associated_animations())
                     dispatch_events_for_animation_if_necessary(animation);
             }
-        });
+        }));
     }
 
     m_animation_driver_timer->start();

Userland/Libraries/LibWeb/DOM/Document.h

@@ -828,7 +828,7 @@ private:
 
     // https://www.w3.org/TR/web-animations-1/#pending-animation-event-queue
     Vector<PendingAnimationEvent> m_pending_animation_event_queue;
-    RefPtr<Platform::Timer> m_animation_driver_timer;
+    RefPtr<Core::Timer> m_animation_driver_timer;
 
     bool m_needs_to_call_page_did_load { false };