LibWeb+WebContent: Change event loop to synchronously paint next frame

...instead of scheduling repaint timer in PageClient.

This change fixes flickering on Discord that happened because:
- Event loop schedules repainting by activating repaint timer
- `Document::tear_down_layout_tree()` destroys paintable tree
- Repaint timer invokes callback and renders an empty frame because
  paintable tree was destroyed

Userland/Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp

@@ -327,7 +327,10 @@ void EventLoop::process()
         if (navigable && navigable->needs_repaint()) {
             auto* browsing_context = document.browsing_context();
             auto& page = browsing_context->page();
-            page.client().schedule_repaint();
+            if (navigable->is_traversable()) {
+                VERIFY(page.client().is_ready_to_paint());
+                page.client().paint_next_frame();
+            }
         }
     });