LibWeb: Make requestAnimationFrame() work in multi-process mode

Since we don't have a direct connection to WindowServer, this is slighly
more naive implementation than what we were doing for single-process
mode.

Basically, there's a global RequestAnimationFrameDriver object that
has a 16ms single-shot timer. Whenever someone registers for a RAF
callback, we start the timer (if needed).

This is not ideal, but it's better than nothing. :^)

Userland/Libraries/LibWeb/DOM/Window.h

@@ -19,6 +19,8 @@
 
 namespace Web::DOM {
 
+class RequestAnimationFrameCallback;
+
 class Window final
     : public RefCounted<Window>
     , public EventTarget {
@@ -90,6 +92,8 @@ private:
     NonnullOwnPtr<HighResolutionTime::Performance> m_performance;
     NonnullRefPtr<CSS::Screen> m_screen;
     RefPtr<Event> m_current_event;
+
+    HashMap<i32, NonnullRefPtr<RequestAnimationFrameCallback>> m_request_animation_frame_callbacks;
 };
 
 }